Rust Converter Not Turning Black?

You've brushed it on, waited, and the rust converter isn't turning black. Or it's gone black in some places and stayed orange in others. It's one of the most common questions we get, and it's nearly always down to the surface, the weather or the metal rather than the product. With Rust Halt™, treated rust normally turns black within about 20 minutes. If yours hasn't, work through these seven causes.

Why does rust converter turn black at all?

The black is the reaction. A rust converter changes rust (iron oxide) into a different, stable compound. With a phosphoric converter like Rust Halt, that's iron phosphate, which dries to a hard black layer you can paint over. So when it turns black, you're watching it work. When it doesn't, the reaction isn't happening, and the question is why.

1. There isn't enough rust

A converter needs rust to react with. If you've sanded the metal back to bright, shiny steel, there's nothing to convert, so it won't change colour. That's not a failure; it just means that bit didn't need converting. Paint bright metal with a suitable metal primer.

2. Grease, oil, wax or silicone on the surface

This is the most common cause. Road film, old wax, oil from your hands or a silicone spray all form a barrier between the converter and the rust. Degrease the area properly, let it dry, and reapply.

3. Thick loose scale still in place

If there's a crust of flaky rust on top, the converter reacts with the loose outer layer, and that layer just falls off later. Wire-brush the loose scale off first. You want firmly attached rust, not crumbs.

4. It's cold or damp

Chemistry slows down in the cold. On a winter's day outdoors it can take a couple of hours to turn black instead of 20 minutes, and damp air slows drying too. Work in a garage if you can, on a dry day, and give it longer before you decide it hasn't worked.

5. The metal isn't iron or steel

Rust converters only work on ferrous metal. Aluminium, stainless steel, chrome and galvanised surfaces won't turn black, because there's no iron oxide to convert. White powdery corrosion on aluminium or zinc is a different problem altogether.

6. There's old paint or primer over the rust

If the rust is under a film of paint, primer or underseal, the converter can't reach it. Scrape or sand through to the rust itself.

7. The product's been contaminated

If you've been dipping a rusty brush straight into the container, rust and dirt get into the liquid and use up its strength. Always pour what you need into a separate pot and work from that. Never pour the leftovers back.

It's gone black and sticky

That's normally too much product. A heavy coat pools in corners and takes much longer to dry. Leave it somewhere warm and dry to finish curing, and next time apply two thin coats instead.

It turned black but the rust has come back

A converter treats the rust; it doesn't waterproof the metal on its own for ever. If the treated area was left unpainted for a long time, moisture gets back in. Paint over it once it's dry. With Rust Halt that's after 3 hours. Our guide to painting over rust converter explains what to use.

Frequently asked questions

How long should rust converter take to turn black?
Rust Halt usually turns rust black in about 20 minutes. In cold, damp weather it can take a couple of hours.

Should I put more on if it hasn't turned black?
Check the prep first: grease, loose scale and old paint are the usual causes. Fix those, then apply a second thin coat.

Do all rust converters turn black?
Most do, but the shade varies between brands and chemistries. Rust Halt turns rust black as it converts.

Can I paint it if part of it is still orange?
Treat the orange areas again first. Painting over unconverted rust lets it carry on spreading under the paint.