Metal Primer:
Which Type for Steel,
Galvanized & Aluminum

The right metal primer depends on three things: which metal you are coating, how well you can clean it, and what goes on top. Get one of those wrong and the best topcoat money can buy will peel. We make primers for steel, galvanized and aluminum that go under our polyurea and polyurethane systems, and we see a steady stream of failed jobs where the topcoat was fine and the primer was the wrong chemistry for the metal. This guide sorts metal primers by type, explains what each one is for, covers the traps on galvanized and aluminum, and includes a primer selector and a primer window planner that checks dew point and tells you when you can topcoat.

Types of metal primer compared

Each row is a real product from a published data sheet, chosen as a typical example of its type. Sort by film thickness, minimum recoat or minimum temperature to see how differently these behave. Your product may differ, so read its sheet.

Priming steel, galvanized and aluminum

Steel and iron

Steel is the easy one, as long as it is clean. Bright steel from a grinder or blast will start flash rusting in humid air, so prime the same day you prep. For paint in mild exposure, an alkyd or DTM acrylic primer over SSPC-SP 3 power tool cleaning is a reasonable system. For long life outdoors, abrasive blast to SSPC-SP 6 or SP 10 and use an epoxy or zinc-rich primer. Sherwin-Williams' recoatable epoxy calls for SP 6 as a minimum and recommends SP 10 for exactly that reason. If the steel is already rusty, see our guides to painting over rust and rust converter, because rusty steel needs a different primer than clean steel.

Galvanized steel

Galvanizing is the one that catches people. Zinc is alkaline, and KTA-Tator warns that oil-based paints built on alkyd esters, epoxy esters or linseed oil can saponify on it: the binder reacts with the zinc and turns to a soft soap at the interface, and the paint sheets off once it gets wet. Rust-Oleum's own enamels and primers say not to use them on galvanized steel.

The AGA splits galvanizing into three conditions. New galvanizing, less than 48 hours old, is clean and takes a wash primer or acrylic pretreatment. Partially weathered galvanizing, from two days to about a year, is the hardest to paint because loose zinc oxides and hydroxides are forming; it needs cleaning and usually a sweep blast. Fully weathered galvanizing, after about a year, has a stable zinc carbonate film and needs only mild cleaning. ASTM D6386 details the steps. A sweep blast should take off no more than about 1 mil of zinc and leave a 1 to 2 mil profile, using a soft abrasive at 20 to 40 psi, and KTA says to paint within 30 minutes.

Aluminum

Aluminum does not rust, but it grows a hard oxide film within minutes of being sanded, and most paints do not stick to that film well. Abrade it, clean it, and prime right away with a self-etching primer or an epoxy primer listed for aluminum. Transtar's 2K self-etch goes on at about 0.5 mil per coat in 2 to 3 coats and accepts topcoats after a 45 minute flash. Do not put polyester body filler over self-etch primer; that data sheet says so directly.

How to prime metal so the topcoat stays on

1. Solvent clean first (SSPC-SP 1)

   Remove oil, grease and shop dirt before you grind or blast. Abrading an oily surface just drives the oil into the profile.

2. Prep to the primer's minimum, or better

   Wire brush and scrape for alkyds, power tool clean for DTM acrylics, blast for high-performance epoxies and zinc. Scuff glossy old paint. On galvanizing, follow ASTM D6386 for its age.

3. Blow off and wipe

   Dust is a bond breaker. Blow off with clean, dry air and wipe with a tack rag or the solvent the primer sheet names.

4. Check the weather on the steel, not in the air

   Use the planner above. Steel in the shade, in the morning or on a trailer that sat outside overnight can be colder than the air and closer to the dew point than you think.

5. Apply at the specified thickness

   More primer is not better with etch and wash primers; KTA notes wash primer over 0.5 mil delaminates. High-build epoxies need their full 4 to 6 mils. Check wet film with a gauge if you have one.

6. Topcoat inside the window

   Too soon and solvent gets trapped; too late and the topcoat has nothing to bite. Fast primers like UltraPrime close in 2 hours. Epoxies can stay open for a long time, but scuff and clean any primer that has sat outdoors.

Metal primer questions

Sources

1. KTA-Tator, how to prepare galvanizing for painting: kta.com/preparing-galvanizing-painting
2. American Galvanizers Association, preparing HDG for paint: galvanizeit.org (preparing HDG for paint)
3. Sherwin-Williams, Recoatable Epoxy Primer B67 product data sheet: paintdocs.com (S-W PDS)
4. PPG, Pitt-Tech Plus EP DTM acrylic primer product data sheet: max.ppg.com (PPG PDS)
5. Transtar, 6111/6114 self-etching primer technical data: tat-co.com (Transtar TDS PDF)
6. Rust-Oleum, Stops Rust Primer Brush technical data: taindustrial.com (Rust-Oleum TDS PDF)
7. Rust-Oleum, Stops Rust Enamel Brush technical data SRT-02: assets.unilogcorp.com (Rust-Oleum TDS PDF)
8. DeFelsko, measuring environmental conditions (dew point and humidity limits): defelsko.com/resources/measuring-environmental-conditions
9. Belzona, guide to ISO and SSPC/NACE surface preparation standards: blog.belzona.com