Everything that happens before the first coat goes on — degreasing, sandblasting, chemical stripping, outgassing, and pretreatment — broken down by metal type with step-by-step procedures for DIY home coaters and professional shops alike.
The #1 Rule of Powder Coating: Your finish is only as good as your prep. No gun, no powder, and no oven can fix a poorly prepared surface. If prep fails, powder fails. Every pinhole, blister, chip, and adhesion failure you’ve ever seen almost certainly started here.
In This Guide
- The Five Pillars of Surface Prep
- Step 1: Degreasing & Cleaning
- Step 2: Chemical & Mechanical Stripping
- Step 3: Sandblasting — Media, Profiles & Technique
- Step 4: Outgassing Pre-Bake
- Step 5: Chemical Pretreatment & Conversion Coatings
- The Water Sheen Test
- Metal Guide: Mild Steel & Carbon Steel
- Metal Guide: Cast Iron
- Metal Guide: Sheet & Extruded Aluminum
- Metal Guide: Cast Aluminum
- Metal Guide: Stainless Steel
- Metal Guide: Galvanized Steel
- Metal Guide: Copper & Brass
- Metal Guide: Chrome-Plated Parts
- Quick Reference: All Metals at a Glance
- FAQ
The Five Pillars of Surface Preparation
Every powder coating job — whether you’re a first-timer in a garage or running a production line — involves some combination of these five preparation steps. Not every part needs every step, but understanding all five lets you build the right prep process for any metal and any application.
| Step | What It Does | When It’s Needed |
|---|---|---|
| 1. Degreasing | Removes oils, grease, fingerprints, and surface contaminants | Always — every part, every time |
| 2. Stripping | Removes old paint, powder, rust, mill scale, and heavy oxidation | Any part with existing coatings or heavy contamination |
| 3. Sandblasting | Creates a mechanical profile (tooth) for the powder to grip and removes remaining surface impurities | Recommended for all metals; essential for castings and rusty/scaled parts |
| 4. Outgassing | Drives trapped oils, moisture, and gasses out of porous metals by pre-baking at elevated temperature | Cast iron, cast aluminum, die-cast zinc, galvanized steel, and any part from a greasy/oily environment |
| 5. Pretreatment | Applies a chemical conversion coating (iron phosphate, zinc phosphate, or zirconium) that improves adhesion and corrosion resistance | Recommended for all metals; essential for exterior or high-durability applications |
The order matters. The standard sequence is: Degrease → Strip (if needed) → Blast → Outgas (if needed) → Pretreat → Powder coat. Skipping a step or doing them out of order will compromise your finish.
Step 1: Degreasing & Cleaning
Degreasing is the foundation. If oils, grease, wax, mold release agents, or fingerprints remain on the surface, they will vaporize through the powder during cure — causing fish eyes, pinholes, craters, and adhesion failure. No amount of sandblasting or pretreatment can fix a part that wasn’t properly degreased.
DIY / HOME SHOP Degrease
- Initial wipe-down — Wipe the part with a clean shop rag dampened with acetone or lacquer thinner to remove heavy grease and oil. Use a fresh section of the rag with each pass to avoid spreading contaminants.
- Alkaline soak/scrub — Mix a quality degreaser like DEP Purple at the concentration recommended on the label in a basin or tub of hot water (120–140°F for best results). Soak the part for 10–20 minutes, then scrub all surfaces with a stiff nylon brush. Pay extra attention to crevices, weld areas, and bolt holes.
- Rinse thoroughly — Rinse with clean water. For small parts, a dedicated rinse tub works well. For larger parts, a pressure washer on a fan setting is effective.
- Water sheen test — Spray clean water over the part and watch how it behaves. If the water flows in an unbroken sheet across the surface, the part is clean. If it beads up anywhere, contaminants remain — go back to step 2 and repeat.
- Handle with nitrile gloves from this point forward. Bare hands will leave oils on the clean surface.
⚠ SILICONE WARNING: Before using any degreaser, check the label for silicone (also listed as silicone additives, dimethicone, or polydimethylsiloxane). Silicone is a powder coating killer — even trace amounts will cause fish eyes, craters, and adhesion failure that cannot be fixed without stripping the part and starting over. Silicone contamination can spread to your blast media, your oven, your gloves, and your shop rags, making it nearly impossible to fully eliminate once introduced. If the product contains silicone in any form, do not use it. DEP Purple is silicone-free, which is why we recommend it.
PRO / PRODUCTION Degreasing Procedure
- Alkaline wash stage — Immerse parts in a heated alkaline wash bath (130–160°F) using a commercial-grade alkaline cleaner formulated for your primary substrate. Agitate or use spray nozzles for 3–5 minutes.
- Rinse stage 1 — Immerse or spray with clean water to remove alkaline residue. Use overflow rinse tanks for consistency.
- Acid etch/clean (if applicable) — For aluminum or mixed-metal work, an acidic cleaner stage removes oxidation and provides a light etch. Typically phosphoric acid-based.
- Rinse stage 2 — Clean water rinse to remove acid residue.
- Water sheen test — Verify clean surface before proceeding to pretreatment stages.
- Note: In production environments, washing is often handled by a multi-stage spray washer or immersion wash line. Chemical concentrations, temperatures, and dwell times should be monitored per your chemical supplier’s specifications.
Step 2: Chemical & Mechanical Stripping
If the part has existing paint, powder coating, rust, mill scale, weld spatter, or heavy oxidation, it must be removed before you can create a proper profile. There are two main approaches — and many jobs use both.
Chemical Stripping
Chemical strippers dissolve existing coatings without altering the base metal profile. They’re ideal when you need to strip a part without the aggressive material removal of blasting — especially on thin metals or parts with fine detail.
| Stripper Type | Best For | Notes |
|---|---|---|
| Methylene chloride-based | Paint, powder, primer on steel | Fast acting (15–60 min); strong fumes — use outdoors with full PPE. Being phased out in many areas. |
| NMP-based (N-Methyl-2-pyrrolidone) | Powder coat, paint on most metals | Slower (2–24 hrs) but safer. Low odor. Works well as a soak. |
| Sodium hydroxide (lye bath) | Paint and powder on steel & iron | Hot lye tank (160–180°F) strips coatings in 1–4 hours. Do NOT use on aluminum — lye dissolves aluminum. |
| Citric acid / phosphoric acid | Light rust, flash rust, oxidation | Gentle on base metal; effective on light surface rust. |
| Media blasting | Everything — coatings, rust, scale | See sandblasting section below for profiles and media selection. |
Never use lye (sodium hydroxide) on aluminum. Lye will aggressively dissolve aluminum, destroying the part. For aluminum stripping, use aircraft-grade paint stripper (methylene chloride or NMP-based) or media blasting only.
Mechanical Stripping
For heavy rust, mill scale, weld scale, and stubborn coatings, mechanical methods often work faster than chemicals. These include wire wheel/cup brushes, flap discs, angle grinder stripping discs, and media blasting. Sandblasting is covered in detail in the next section. For hand tools, work to bare, clean metal — but be aware that grinders can leave embedded abrasive particles and heat-affected zones that should be addressed before coating.
Step 3: Sandblasting — Media, Profiles & Technique
Sandblasting (media blasting) is the single most effective way to prepare a surface for powder coating. It simultaneously removes contaminants and creates a mechanical profile — a pattern of microscopic peaks and valleys that gives the powder something to grip. The profile you create depends on the media type, grit size, blast pressure, and distance from the part.
Media Selection Guide
| Media | Grit Range | Profile | Best Metals | Notes |
|---|---|---|---|---|
| Aluminum Oxide | 60–150 | Aggressive, angular | Steel, iron, aluminum, stainless | The all-purpose workhorse. Reusable. Creates an excellent anchor profile. |
| Glass Bead | 70–170 | Smooth, peened | Aluminum, stainless, soft metals | Cleans and peens without cutting. Satin finish. Gentler on thin metals. |
| Silicon Carbide | 60–120 | Very aggressive, sharp | Cast iron, heavy scale on steel | Hardest common abrasive. Cuts fast. Good for heavy mill scale and casting skin. |
| Crushed Walnut Shell | 12–40 mesh | Very light | Soft metals, chrome removal | Will not damage soft surfaces. Good for removing chrome plating. |
| Steel Grit / Shot | S110–S330 | Aggressive, rounded | Steel, cast iron | Production-grade. Never use on aluminum or stainless (causes contamination). |
| Garnet | 60–120 | Moderate, angular | Steel, iron | Natural mineral. Good profile, low dust. Single-use. |
| Plastic Media | 12–40 mesh | Very light | Fiberglass, composites, soft alloys | Strips coatings without damaging substrate. |
Recommended Blast Profiles by Metal
| Metal | Recommended Media | Grit | Pressure (PSI) | Target Profile |
|---|---|---|---|---|
| Mild / Carbon Steel | Aluminum oxide | 80–120 | 80–100 | 2.0–3.0 mil (50–75 µm) |
| Cast Iron | Aluminum oxide or silicon carbide | 60–100 | 80–100 | 2.5–3.5 mil |
| Sheet / Extruded Aluminum | Aluminum oxide or glass bead | 100–150 | 40–60 | 1.0–2.0 mil — use lower pressure to avoid warping |
| Cast Aluminum | Aluminum oxide | 80–120 | 60–80 | 1.5–2.5 mil |
| Stainless Steel | Aluminum oxide or glass bead | 100–150 | 60–80 | 1.5–2.5 mil — never use carbon steel media |
| Galvanized Steel | Aluminum oxide (fine) | 120–150 | 30–50 | 0.5–1.5 mil — light sweep only; preserve zinc layer |
| Copper / Brass | Glass bead | 100–170 | 30–50 | 1.0–1.5 mil — gentle, even passes |
| Chrome-plated parts | Aluminum oxide | 80–120 | 80–100 | Remove all chrome to bare metal (2.0–3.0 mil on base) |
Blast Tip: After blasting, blow the part off thoroughly with clean, dry compressed air to remove all residual media dust. Any trapped media — especially in recesses and threads — will cause defects under the powder. Follow up with a tack cloth if accessible.
Cross-contamination warning: Never blast aluminum with media that was previously used on steel. Embedded steel particles in aluminum will rust under the powder coat. Use dedicated media for each metal type, or use aluminum oxide (which is safe for all metals) and keep batches separate.
Step 4: Outgassing Pre-Bake
Outgassing is the process of pre-baking a part at a temperature above the powder’s cure temperature to drive out trapped gasses, oils, moisture, and other volatiles before the powder is applied. When these substances escape during the actual powder cure instead of before it, they erupt through the molten powder layer and create pinholes, bubbles, craters, and blisters that are impossible to fix without stripping and starting over.
Which Parts Need Outgassing?
| Metal / Condition | Outgassing Needed? | Why |
|---|---|---|
| Cast iron | Yes — always | Extremely porous; absorbs oils deeply during manufacturing and use. Often the most persistent outgasser. |
| Cast aluminum | Yes — always | Porous from the casting process. Sand castings are worse than die castings. |
| Die-cast zinc (Zamak/pot metal) | Yes — always | Highly porous and prone to trapped gasses from the die-casting process. |
| Galvanized steel | Yes — always | Zinc coating traps moisture and air. Must pre-bake to release before powder cure. |
| Parts from oily/greasy environments | Yes | Engine parts, machine shop parts, etc. — oils soak deep into the metal over time. |
| Cold-rolled steel (new/clean) | Usually not | Non-porous. A proper degrease and blast is typically sufficient. |
| Sheet / extruded aluminum (new) | Usually not | Non-porous. Degrease and blast, then coat. |
| Stainless steel | Usually not | Dense, non-porous. Clean and blast is sufficient for most applications. |
Outgassing Temperature & Time Guide
| Metal | Outgas Temperature | Hold Time (at PMT*) | Max Safe Temp | Notes |
|---|---|---|---|---|
| Cast iron | 425–450°F | 30–60 minutes | 500°F+ | Iron can handle high temps. Continue until smoking stops, then add 10 min. |
| Cast aluminum | 425–430°F | 20–40 minutes | 450°F | Stay under 450°F to avoid risk of annealing. Sand castings need longer. |
| Die-cast zinc | 375–400°F | 30–45 minutes | 420°F | Zinc’s lower melting point demands caution. Go low and slow. |
| Galvanized steel | 400–450°F | 45–60 minutes | 535°F (without phosphate); 535°F max overall | Must reach oven temp throughout part. Thick structural takes longer. |
| Oily steel parts | 400–425°F | 20–30 minutes | No practical limit | Until smoking stops plus 5–10 extra minutes. |
*PMT = Part Metal Temperature. Use an infrared thermometer to verify the actual part (not oven air) has reached temperature before starting the timer.
Important: After outgassing, let the part cool to room temperature before applying powder. Some coaters prefer to coat while still slightly warm to prevent flash rust on ferrous metals — but never apply powder to a hot part, as the powder will pre-gel unevenly and create defects.
The smoke test: While outgassing, watch the part through the oven window (or open briefly to check). If you see smoke or haze rising off the part, oils are still coming out. Keep baking. When the smoking stops, the part is nearly done — give it another 5–10 minutes to be safe.
Step 5: Chemical Pretreatment & Conversion Coatings
Chemical pretreatment creates a thin conversion coating on the metal surface that dramatically improves powder adhesion and corrosion resistance. Think of it as a chemical “glue layer” between the bare metal and the powder. While a good sandblast profile alone will give you mechanical adhesion, adding a chemical pretreatment gives you chemical adhesion as well — and the combination is what separates a finish that lasts 2 years from one that lasts 20.
DIY / HOME SHOP Pretreatment Options
For home and small shop coaters who don’t have immersion wash lines, here are practical pretreatment approaches:
- Option A — Iron phosphate spray/wipe (steel & iron): Products like Ospho, PreKote, or Rust-Oleum Rust Reformer can be sprayed or wiped onto clean, blasted steel. Follow the product’s instructions for dwell time (typically 10–20 minutes), then rinse with clean water and dry in oven.
- Option B — Vinegar etch (aluminum): A 50/50 white vinegar and water bath (10–15 min soak) provides a light acid etch on aluminum. Rinse thoroughly. This is a budget option — not as effective as a commercial etch, but better than nothing.
- Option C — Self-etching primer (any metal): A self-etching aerosol primer provides both adhesion promotion and mild corrosion protection. Apply a thin, even coat and let cure per the can’s instructions. Powder can be applied over the cured primer.
- Option D — Sandblast only (minimum viable prep): If chemical pretreatment isn’t practical, a thorough sandblast to a 2–3 mil profile on clean, degreased metal is the minimum acceptable prep. You’ll get mechanical adhesion, but less corrosion resistance than chemical pretreatment provides. Coat as quickly as possible after blasting — especially on ferrous metals, which can flash-rust within hours.
PRO / PRODUCTION Pretreatment Process
Production environments typically use multi-stage spray washers or immersion tanks. A common 5-stage iron phosphate process looks like:
- Stage 1 — Alkaline clean (130–160°F, 2–3 min)
- Stage 2 — Fresh water rinse (ambient, 1–2 min)
- Stage 3 — Iron phosphate coating (100–150°F, pH 4.0–5.0, 1–2 min)
- Stage 4 — Fresh water rinse (ambient, 1–2 min)
- Stage 5 — Non-chrome seal rinse or DI water rinse (ambient, 30–60 sec)
For zirconium-based pretreatment, stages 3–5 are simplified: zirconium conversion coatings operate at ambient temperature, require no heavy metal waste treatment, and work on both steel and aluminum — making them ideal for mixed-metal production lines.
Pretreatment Types Compared
| Pretreatment | Metals | Corrosion Resistance | Complexity | Best For |
|---|---|---|---|---|
| Iron Phosphate | Steel, iron | Good | Low–moderate | DIY and production. Most common conversion coating. Single-stage or multi-stage. |
| Zinc Phosphate | Steel, iron | Excellent | Moderate–high | Automotive, outdoor, high-corrosion environments. Multi-stage process. |
| Zirconium (Zr) | Steel, aluminum, mixed | Very good | Low–moderate | Modern replacement for phosphate. No heavy metals. Works at room temp. Excellent for mixed-metal lines. |
| Chromate Conversion | Aluminum | Excellent | Moderate | Architectural aluminum. Being phased out due to hexavalent chromium (Cr6+) health concerns. |
| Non-chrome conversion (TCP, etc.) | Aluminum | Very good | Low–moderate | Modern chromate replacement for aluminum. Trivalent chromium or chrome-free. |
| Self-etching primer (spray) | All metals | Moderate | Very low | DIY solution when chemical conversion isn’t practical. Spray-and-go. |
⚗ The Water Sheen Test (Water Break Test) — YOUR MOST IMPORTANT QUALITY CHECK
This is the simplest, most reliable, and most overlooked test in all of powder coating. It takes 10 seconds, costs nothing, and will save you from more failed finishes than any other single practice. Run this test after degreasing and before blasting to confirm your cleaning was effective, and again after pretreatment as a final check. Make it a non-negotiable habit on every single part.
How to perform it:
Spray or pour clean water over the part’s surface. Then watch how the water behaves:
✅ PASS — Water sheets in an unbroken, continuous film across the surface. The part is clean and ready for the next step.
❌ FAIL — Water beads up, breaks into droplets, or pulls away from any area. Oils or contaminants are still present. Go back and degrease again. Do not proceed until the water sheets evenly.
Why it works: Clean metal is hydrophilic (attracts water) — water clings to it in an even sheet. Oils and contaminants are hydrophobic (repel water) — they cause the water to bead up and break. This test is used by hobbyists and billion-dollar production facilities alike because it works, every time. If your water doesn’t sheet, your powder won’t stick.
Metal-by-Metal Preparation Guides
Below are complete step-by-step procedures for each common metal — with a DIY/home shop approach and a pro/production approach for each.
DIY PROCEDURE
- Degrease with Purple Power or SuperClean in hot water. Scrub, rinse, water sheen test.
- Strip any existing coatings or rust by sandblasting with aluminum oxide (80–120 grit) at 80–100 PSI to bare, white metal.
- Blow off all blast dust with clean, dry compressed air.
- If the part came from an oily environment, outgas at 400–425°F for 20–30 min. Otherwise, skip to step 5.
- Optional but recommended: Apply Ospho or similar iron phosphate product per label instructions. Rinse and dry.
- Wipe down with acetone on a clean lint-free cloth (final solvent wipe).
- Powder coat as quickly as possible — steel can flash-rust within hours in humid conditions.
PRO PROCEDURE
- Alkaline wash at 140°F, 2–3 min. Rinse.
- Blast to SSPC-SP6 (commercial blast) or SP10 (near-white) standard with aluminum oxide or steel grit. Profile: 2.0–3.0 mil.
- 5-stage iron phosphate or zirconium conversion process (see pretreatment section).
- Oven dry at 250–300°F for 10–15 min.
- Powder coat immediately after cooling to handling temperature.
DIY PROCEDURE
- Degrease aggressively — hot alkaline soak (Purple Power / SuperClean in hot water) for 20–30 min minimum. Cast iron absorbs oils deeply; you may need to repeat.
- Rinse thoroughly. Water sheen test.
- Blast with aluminum oxide (60–100 grit) at 80–100 PSI. Cast iron’s surface skin is tough — use a coarser grit than you would on sheet steel. Get into every pore and recess.
- Blow off thoroughly with compressed air.
- Outgas at 425–450°F for 30–60 minutes (at PMT). Watch for smoking — continue until all smoking stops, then hold 10 minutes longer. Some old, oil-soaked castings (engine blocks, manifolds) may need multiple outgas cycles.
- Let cool. Apply iron phosphate product or self-etching primer.
- Powder coat. Consider using an Outgas-Forgiving (OGF) primer as your first coat on heavy castings — these primers are formulated with an extended flow-out time that lets remaining gas escape before the coating gels.
PRO PROCEDURE
- Alkaline wash at 150°F, 5 min minimum. Rinse.
- Shot blast or wheel blast with steel grit (S230–S330) or aluminum oxide to remove casting skin and create 2.5–3.5 mil profile.
- Outgas at 425–450°F, 45–60 min at PMT. For production, a dedicated pre-bake oven in the line handles this step.
- Iron phosphate or zirconium conversion coating.
- Apply OGF primer, cure, then topcoat. In production, this two-coat process is standard for cast iron.
Cast iron is the most stubborn outgasser. Some old, oil-saturated castings will never fully stop outgassing. For these parts, an OGF primer is not optional — it’s the only reliable path to a defect-free finish. The primer’s extended gel time lets residual gas escape harmlessly before the surface locks.
Aluminum’s natural oxide layer (aluminum oxide) forms within minutes of exposure to air. This oxide layer must be removed or etched before powder coating — powder adheres to the oxide, and when the oxide flakes, the powder goes with it. Mechanical profiling (blasting) or chemical etching breaks through this layer and provides a fresh, adhesive surface.
DIY PROCEDURE
- Degrease with Purple Power or Simple Green HD in warm water. Rinse. Water sheen test.
- Blast with aluminum oxide (100–150 grit) or glass bead at 40–60 PSI. Aluminum is soft — high pressures will warp thin sheet and embed media. Use even, sweeping passes.
- Blow off thoroughly.
- Optional etch: Soak in 50/50 white vinegar and water for 10–15 min to lightly acid-etch the surface. Rinse well with clean water.
- Dry in oven at 250°F for 10 min (drives off moisture and gives you time before oxide reform).
- Coat within 4 hours of blasting. Aluminum’s oxide reforms quickly — the sooner you coat, the better your adhesion.
PRO PROCEDURE
- Alkaline clean (use a cleaner rated for aluminum — avoid highly caustic formulas that over-etch).
- Acid etch with phosphoric acid-based etch (removes oxide layer and creates micro-profile).
- Non-chrome conversion coating (trivalent chromium phosphate or zirconium). Chromate conversion is still used in some architectural applications but is being phased out due to hexavalent chromium health concerns.
- DI water seal rinse.
- Oven dry at 250°F, 5–10 min.
- Coat immediately.
Never blast aluminum with steel shot or steel grit. Steel particles embed in the soft aluminum surface and will rust under the powder coat, causing brown staining and adhesion failure. Use aluminum oxide, glass bead, or garnet only.
DIY PROCEDURE
- Degrease thoroughly — cast aluminum from engines and drivetrains is often soaked in decades of oil. Hot alkaline soak for 20–30 min, scrub, rinse. Repeat if needed. Water sheen test.
- Blast with aluminum oxide (80–120 grit) at 60–80 PSI. Work all surfaces including inside recesses where casting sand and oils hide.
- Blow off completely. Use a pick or needle to clear media from bolt holes and tight channels.
- Outgas at 425–430°F for 20–40 minutes at part metal temperature. Never exceed 450°F. Watch for smoke — sand castings (like older intake manifolds) will outgas longer than die castings (like most wheels).
- Let cool to handling temperature.
- Optional but recommended: Light acid etch (vinegar bath) or self-etching primer.
- Powder coat. For heavily porous castings, use an OGF primer as your first coat, cure, then apply your topcoat color.
PRO PROCEDURE
- Alkaline clean (aluminum-safe formula), 140–150°F.
- Blast with aluminum oxide (80–120 grit) at controlled pressure.
- Outgas at 425–430°F, 30–40 min at PMT. For production wheel coating, a dedicated outgas oven is standard.
- Acid etch and non-chrome conversion coating (zirconium or TCP).
- OGF primer coat on castings with known porosity. Cure, then topcoat.
Stainless steel’s chromium oxide passive layer protects it from corrosion — but that same layer also resists powder adhesion. Like aluminum, this passive layer must be broken through mechanically (blasting) or chemically (etching) for the powder to properly bond.
DIY PROCEDURE
- Degrease with acetone wipe or alkaline cleaner. Stainless is typically clean from the factory, but machining oils and fingerprints are common. Rinse. Water sheen test.
- Blast with aluminum oxide or glass bead (100–150 grit) at 60–80 PSI. This breaks the passive layer and creates a profile for adhesion.
- Blow off thoroughly.
- Wipe with acetone on a lint-free cloth (final solvent wipe).
- Powder coat within a few hours of blasting.
PRO PROCEDURE
- Alkaline clean and rinse.
- Blast to 1.5–2.5 mil profile with aluminum oxide.
- Zirconium conversion coating (works well on stainless and doesn’t interfere with the passive layer’s benefits). Rinse and dry.
- Coat immediately.
Never blast stainless steel with plain carbon steel media. Carbon steel particles will embed in the surface and rust — ruining the stainless steel’s corrosion resistance and creating brown stains under the powder. Use only aluminum oxide, glass bead, or stainless steel shot.
Galvanized steel is one of the trickiest metals to powder coat. The zinc coating traps moisture and gasses that cause aggressive outgassing during cure, and zinc’s surface chemistry makes adhesion challenging. The goal is to prepare the surface without removing the zinc layer entirely (since the zinc provides the underlying corrosion protection).
DIY PROCEDURE
- Degrease with alkaline cleaner. Avoid harsh solvents that can react with zinc. Rinse well.
- Light blast with aluminum oxide (120–150 grit) at low pressure (30–50 PSI). The goal is to create a light profile and remove zinc oxidation (white rust) — not to strip the zinc layer off. A quick, even sweep is all you need.
- Outgas at 400–450°F for 45–60 minutes at part metal temperature. This is critical — the zinc layer traps significant moisture. If you skip this step, expect widespread pinholes and blistering.
- Let cool. Apply a self-etching primer formulated for galvanized surfaces.
- Powder coat as quickly as possible after outgassing and priming. Zinc re-oxidizes rapidly.
PRO PROCEDURE
- Alkaline clean (zinc-safe formula). Rinse.
- Light sweep blast (120–150 grit aluminum oxide, 30–50 PSI) to remove white rust and create profile. Preserve zinc layer.
- Pre-bake at 400–450°F for 45–60 min at PMT.
- Phosphoric acid-based etch (removes remaining zinc oxide) followed by a phosphate or zirconium conversion coating formulated for galvanized substrates.
- Coat immediately after cooling. Use a zinc-compatible primer if required by the application.
Temperature limits: If the galvanized part has already received a phosphate pretreatment, do not pre-bake above 535°F — higher temperatures will convert the phosphate coating into a powdery residue that destroys adhesion.
Copper and brass are soft metals that oxidize (patina) readily. Their surface oxides are tenacious and must be removed for proper adhesion. These metals also have excellent thermal conductivity, so they heat and cool faster than steel — adjust your cure monitoring accordingly.
DIY PROCEDURE
- Degrease with acetone wipe or alkaline cleaner soak. Rinse. Water sheen test.
- Remove all tarnish/patina. For light oxidation, a phosphoric acid-based cleaner (like Naval Jelly on brass) works well. For heavy patina, blast with glass bead (100–170 grit) at 30–50 PSI. Copper and brass are soft — high pressure will damage the surface.
- Blow off thoroughly.
- Apply self-etching primer or coat immediately after cleaning. Copper re-oxidizes quickly.
PRO PROCEDURE
- Alkaline clean (mild formula for soft metals). Rinse.
- Acid etch with citric or phosphoric acid-based solution to remove oxide layer and create micro-profile. Rinse.
- Non-chrome conversion coating or self-etch primer.
- Coat immediately — these metals re-oxidize within minutes.
Chrome must be completely removed before powder coating. Powder will not adhere to chrome plating — the surface is too smooth and chemically inert. Even a tiny spot of remaining chrome will become an adhesion failure point.
DIY PROCEDURE
- Degrease the entire part. Chrome is usually clean, but grease hides in seams and mounting points.
- Blast with aluminum oxide (80–120 grit) at 80–100 PSI until ALL chrome, nickel, and copper layers are removed and you’re down to bare base metal. This takes time — chrome is hard. Verify by visual inspection. Bare steel is gray; remaining chrome/nickel is bright and reflective. If it shines, it’s not done.
- Blow off thoroughly.
- If the base metal is cast (like a die-cast trim piece), outgas per the appropriate casting procedure above.
- Apply iron phosphate treatment or self-etching primer (you’re now treating the exposed base metal).
- Powder coat.
PRO PROCEDURE
- Chemical strip chrome plating using a commercial chrome stripper (typically hydrochloric acid-based for the chrome layer, then a separate nickel/copper stripper). This is faster than blasting for production volumes.
- Blast exposed base metal to proper profile.
- Standard pretreatment for the base metal type (usually iron phosphate for steel).
- Coat.
Alternative for DIY: If chemical chrome stripping isn’t practical, walnut shell media at moderate pressure can remove chrome plating without excessively cutting the base metal. It’s slower than aluminum oxide but gives you more control. Follow up with a quick pass of aluminum oxide for profile.
Quick Reference: All Metals at a Glance
| Metal | Blast Media & Grit | Blast PSI | Outgas? | Outgas Temp / Time | Pretreatment (DIY) | Pretreatment (Pro) |
|---|---|---|---|---|---|---|
| Mild Steel | Alum. oxide 80–120 | 80–100 | If oily | 400–425°F / 20–30 min | Ospho / iron phosphate spray | Iron or zinc phosphate line |
| Cast Iron | Alum. oxide 60–100 | 80–100 | Always | 425–450°F / 30–60 min | Iron phosphate + OGF primer | Iron phosphate + OGF primer |
| Sheet Aluminum | Alum. oxide 100–150 or glass bead | 40–60 | No | — | Vinegar etch or self-etch primer | Acid etch + non-chrome conversion |
| Cast Aluminum | Alum. oxide 80–120 | 60–80 | Always | 425–430°F / 20–40 min | Self-etch primer + OGF if needed | Acid etch + conversion + OGF primer |
| Stainless Steel | Alum. oxide or glass bead 100–150 | 60–80 | No | — | Blast profile only | Zirconium conversion |
| Galvanized Steel | Alum. oxide 120–150 | 30–50 | Always | 400–450°F / 45–60 min | Self-etch primer for galv | Phosphoric etch + conversion |
| Copper / Brass | Glass bead 100–170 | 30–50 | If oily | 400°F / 15–20 min | Self-etch primer | Acid etch + conversion |
| Chrome Plated | Alum. oxide 80–120 | 80–100 | If cast base | Per base metal | Remove chrome → treat base metal | Chemical strip → treat base metal |
Frequently Asked Questions
What is the water sheen test in powder coating?
The water sheen test (also called the water break test) is the simplest way to verify a surface is clean. Pour or spray clean water over the part. If the water flows in an unbroken sheet (sheeting), the surface is clean. If it beads up, contaminants remain and you need to degrease again.
Do I need to outgas every part before powder coating?
No. Outgassing is primarily for porous castings (cast iron, cast aluminum, die-cast zinc), galvanized steel, and any part that lived in a greasy environment. Cold-rolled sheet steel, sheet aluminum, and stainless steel typically don’t need outgassing. When in doubt, outgas — it only adds time, and skipping it on a part that needs it will ruin your finish.
What sandblast media should I use for powder coating?
Aluminum oxide (80–120 grit) is the all-purpose choice that works safely on all metals. Glass bead is gentler and preferred for soft metals like aluminum, copper, and brass. Silicon carbide is the most aggressive and best for heavy scale on cast iron. Never use carbon steel media on aluminum or stainless steel — it causes cross-contamination and rust.
What temperature should I outgas cast aluminum?
425–430°F at part metal temperature (measured with an infrared thermometer, not oven air temp). Hold for 20–40 minutes after the part reaches temperature. Never exceed 450°F to avoid any risk of weakening the aluminum. Continue until all visible smoke stops, then hold another 5–10 minutes.
What is iron phosphate pretreatment and do I need it?
Iron phosphate is a chemical conversion coating that creates a thin crystalline layer on steel. It improves both adhesion and corrosion resistance. For DIY work, it’s not strictly required if you sandblast to a good profile — but adding it significantly improves durability, especially for outdoor parts. For professional work, it’s considered standard practice.
Can I powder coat galvanized steel?
Yes, but it requires careful preparation. You must pre-bake galvanized parts at 400–450°F for 45–60 minutes to drive out trapped moisture. A light blast with fine media (120–150 grit, low pressure) creates a profile without stripping the zinc. Use a self-etching primer formulated for galvanized surfaces for best adhesion.
How quickly do I need to powder coat after sandblasting?
As soon as practically possible. Bare steel can begin flash-rusting within hours in humid conditions. Aluminum reforms its oxide layer within minutes. Ideally, coat within 2–4 hours of blasting. If you can’t coat the same day, store parts in a dry, climate-controlled area and consider a quick re-blast or solvent wipe before coating.
What is an OGF (Outgas-Forgiving) primer?
OGF primers are specialty powder coating primers formulated with an extended flow-out time. During cure, they stay liquid longer than standard powders, allowing any remaining trapped gasses to escape through the molten primer before it gels and hardens. This prevents the pinholes and bubbles that outgassing causes. OGF primers are especially important for cast iron and heavily porous cast aluminum parts. Apply the OGF primer as your first coat, cure it, then apply your color topcoat.
Remember: There are no shortcuts to surface prep. Every blister, chip, pinhole, and adhesion failure can be traced back to a prep step that was skipped, rushed, or done incorrectly. Invest the time here and your powder coating results will speak for themselves.
Have questions about surface preparation for a specific project or metal type? Contact us — we’re here to help you get it right.