Prime bare steel or iron with a metal primer, then topcoat: Benjamin Moore says priming gives rust inhibition and topcoat adhesion, and Krylon says exterior metal should always be primed. Pick the primer by how rusty the metal is: Rust-Oleum makes Clean Metal Primer for bare or lightly rusted metal, Rusty Metal Primer for heavily rusted metal, and Rust Reformer to convert rust.
First: is it ferrous?
Krylon’s metal how-to says “Anything with iron content is considered a ferrous metal. The quickest way to test is if the metal responds to a magnet.” Benjamin Moore’s metal guide lists steel, cast iron and wrought iron as ferrous, “magnetic and susceptible to rust,” and puts aluminum, copper, stainless steel and zinc in the non-ferrous group. It also says to treat galvanized metal as non-ferrous.
This page is about ferrous metal. For the others, see paint for galvanized metal, paint for aluminum and paint for stainless steel.
Which primer for how much rust
| Product | For | Not for | Dry and recoat |
|---|---|---|---|
| Rust-Oleum Stops Rust Clean Metal Primer (brush) | “bare, lightly rusted or previously painted metal” | Galvanized steel | Touch in 2 to 4 hours; recoat after 24 hours |
| Rust-Oleum Stops Rust Clean Metal Primer (spray) | “lightly rusted, bare metal, or previously painted surfaces” | Galvanized steel | Touch in 15 to 30 minutes; topcoat or recoat within 1 hour or after 48 hours |
| Rust-Oleum Stops Rust Rusty Metal Primer | “heavily rusted metal”; it “Bonds tightly to rust” | Galvanized steel | Brush: recoat after 24 hours; spray: within 1 hour or after 48 hours |
| Rust-Oleum Stops Rust Rust Reformer | Converts rust “into a smooth, paintable surface”; “Do not use on non-rusted surfaces.” | Galvanized steel; rust-free metal | Aerosol: topcoat after 24 hours |
| Rust-Oleum Automotive Self Etching Primer | “bare metal, aluminum and fiberglass” | Galvanized metal | Touch in 10 minutes; topcoat after 30 minutes |
| Krylon Rust Tough Primer (aerosol) | “Metal, Steel, Aluminum”; “can be applied directly to metal surfaces” | Not stated | Touch in 25 minutes (Krylon’s FAQ says 20); handle in 1 hour |
| Krylon Rust Tough Brush-On Primer | “Metal, Aluminum, Steel” | Not stated | Touch in 4 hours; handle in 8 hours |
| Krylon Rust Tough Rust Converter | “Spray directly over rust” | Not stated | Touch in 2 hours, handle in 3; Krylon’s FAQ says lightly sand after 24 hours before a color topcoat |
| Sherwin-Williams All Surface Enamel Latex Primer | “iron, steel, aluminum, and galvanizing” | Not stated | Touch in 1 hour; recoat in 4 hours; apply above 50°F |
| Benjamin Moore HP1320 Universal Metal Primer | Ferrous metal, “new or properly prepared rusted surfaces” | “non-ferrous metals such as galvanized, aluminum”; immersion | Touch in 30 minutes; recoat in 4 hours |
| Benjamin Moore HP1100 Acrylic Metal Primer | “steel, iron, and non-ferrous metals”, and “tightly adhering rust” | Floors and immersion service; don’t apply below 50°F | Touch in 30 minutes; recoat in 2 hours |
From each product’s data sheet or page. Rust-Oleum disagrees with itself on Rusty Metal Primer: its spray data sheet says it is “designed for heavily rusted surfaces only,” and its page says to use Clean Metal Primer on clean or lightly rusted metal, but the brush data sheet says to use it “for sound rusted or clean metal.” We’d follow the page: Clean Metal Primer on clean metal.
Krylon Rust Protector no longer has a page on Krylon’s site that we could find; the rust-preventive pages we read there are Rust Tough, and the Rust Tough Enamel page compares itself with Rust Protector. That enamel says “Direct to metal application - no primer needed.” Rust-Oleum’s Stops Rust enamel data sheet, on the other hand, says a Stops Rust primer is “especially recommended for bare wood and metal.”
Prep, in the makers’ words
- Rust-Oleum (Stops Rust brush data sheet): wash off dirt, grease, oil and salt with a commercial detergent, rinse and let dry, then “Scrape and wire brush or power tool clean to remove loose rust, scale and deteriorated previous coatings.” Glossy old finishes should be sanded.
- Sherwin-Williams All Surface Enamel Latex Primer: “Rust and mill scale must be removed using sandpaper, wire brush, or other abrading method. Bare steel must be primed the same day as cleaned.”
- Benjamin Moore’s metal guide: clean with a detergent or a solvent like denatured alcohol, remove rust with a wire brush, then wipe the dust off with a damp cloth. Its FAQ says metal “does not necessarily need to be sanded, but it does need to be prepared.”
- Krylon’s FAQ: “The surface should be clean, dull and dry.” It warns that strong solvents on old paint can make the new paint wrinkle or pull away.
Weather and the recoat window
Rust-Oleum’s Stops Rust brush primers go on between 50 and 90°F with humidity below 85%, and its spray primers want humidity below 65%. The spray primers’ data sheet gives a window: topcoat within 1 hour or after 48 hours. Rust-Oleum’s Stops Rust enamel spray data sheet says “Primers may be top coated immediately.” We’d follow the primer’s own data sheet and not topcoat between 1 and 48 hours.
Krylon’s FAQ says “Do not sand old paint that may contain lead.” EPA says that if your home was built before 1978, it is more likely to have lead-based paint. If you sand or scrape old paint yourself, EPA says to use its lead-safe work practices, and not to attempt to remove lead-based paint yourself. See epa.gov/lead.
Rust-Oleum’s Clean Metal Primer (brush) SDS lists “H226 Flammable liquid and vapour” and “H360 May damage fertility or the unborn child.” The spray version’s SDS lists “H222 Extremely flammable aerosol” and “H351 Suspected of causing cancer.” Rust Reformer spray’s SDS says “Use only outdoors or in a well-ventilated area.”