Rust Converters Compared: Tannic Acid vs. Phosphoric Acid vs. Rust-Encapsulating Primers
This guide contains affiliate links. As an Amazon Associate I earn from qualifying purchases. Disclosure.
A rust converter chemically changes rust (iron oxide) into a more stable compound you can paint over. It's for when you can't practically get down to bare metal. It is not a fix for rust that has eaten through the metal.
We haven't run our own lab tests. This guide explains product types and how to use them, based on their chemistry and manufacturer documentation. Always follow the specific product's data sheet.
The main types
| Type | How it works | Good for | Watch out for |
|---|---|---|---|
| Tannic-acid converters (often with a polymer) | Tannins react with iron oxide to form a dark, stable iron tannate; the polymer forms a primer-like film | Car frames, trailers, fences, equipment with firmly attached rust | Needs some rust to react with; usually must be topcoated for UV/weather resistance |
| Phosphoric-acid treatments | Convert rust to iron phosphate and lightly etch steel | Prep before priming; light rust | Residue must be handled per label (some require rinsing); acidic, wear PPE |
| Rust-encapsulating / rust-inhibiting primers | Seal and isolate rust from moisture and oxygen rather than converting it | Tight rust where conversion isn't practical | Not a converter; loose rust must still be removed |
How to get a converter to last
- Remove loose, flaking rust and scale with a wire brush. Converters can't bond to flakes that fall off later.
- Degrease and let it dry.
- Apply thin coats within the temperature and humidity range on the label.
- Wait the full cure time (it usually turns black), then topcoat with a compatible paint.
Convert or remove?
For structural or high-value parts, and anything going under a high-performance coating, removing rust down to clean metal (blasting, laser or mechanical) is still the better base. Converters are a practical choice for big, hard-to-strip items.
Products for this job
- Rust-Oleum Stops Rust Rust Reformer spray Aerosol converter for smaller rusted areas. Check price on Amazon
- Corroseal water-based rust converter & primer (1 gallon) Brush-on converter/primer for big items like trailers. Check price on Amazon
- Phosphoric acid metal prep Light-rust treatment and etch before priming. Check price on Amazon
- Rust encapsulator primer Seals tight rust where conversion isn't practical. Check price on Amazon