How to Treat Rust on a Car

How to treat rust on a car depends on how far it's gone. Surface rust and the early bubbling under the paint on a sill or arch can be stopped at home in a weekend: cut back, convert the rust with something like Rust Halt™ Rust Converter Gel Spray, and repaint. Rust that's eaten through the metal can't be treated. It has to be cut out and welded. Here's how to tell the difference, and how to do the treatable kind properly.

The three stages of car rust

• Surface rust: orange staining where paint's been chipped, usually from stones, on sills, arches and the bottom of doors. The metal is still solid. This is the easy one.
• Bubbling or scale: paint lifting in blisters, with flaky rust underneath. It's worse than it looks from outside, because the rust has been spreading under the paint, but if the steel under the scale is sound it can still be treated.
• Rot: holes, crumbling edges, or metal you can push a screwdriver through. No rust treatment will fix it, because there's no steel left to treat.

How to treat rust on sills and wheel arches

This is where most cars rust first, because it's where the water and road salt sit. Pick a dry day, or work in a garage.

1. Cut back to sound paint. Use a scraper and abrasive paper to open up any bubbles. Keep going until you reach paint that's firmly stuck and metal that isn't flaking. It's always a bigger area than you expect.
2. Wire-brush the loose rust. Get the flaky scale off. You don't need bright metal; the converter deals with the rust left in the pits.
3. Clean and degrease. Road film, wax and silicone stop a converter reacting. Wipe the area down and let it dry.
4. Mask around the repair. Keep the converter off good paint, rubber seals, chrome and glass.
5. Apply Rust Halt. The gel spray clings to vertical panels like sills and door bottoms, so it stays on the rust instead of running off. Two thin coats beat one thick one.
6. Let it convert. The rust turns black in about 20 minutes. In cold weather it can take longer.
7. Prime, fill and paint. After 3 hours the converted surface is ready for paint. Fill any pitting, then prime and colour-match as you would on any repair.

Treating chassis and underbody rust

Underneath the car, appearance doesn't matter as much as protection. The method is the same, with a few differences:

• Get the car safely up on axle stands or ramps. Never work under a car supported only by a jack.
• Scrape off any old underseal that's lifting. Rust spreads underneath loose underseal where you can't see it.
• Wire-brush, degrease, then treat with Rust Halt. For big areas like a chassis, the 5 litre trade size brushed on is more economical than spraying.
• Once it's converted and dry, finish with chassis paint or a fresh underbody coating.

When is it too far gone?

Be honest with yourself here. If the metal flexes when you press it, crumbles at the edges, or has holes in it, it needs cutting out and new metal welding in. A rust converter will stop rust spreading on the good metal around a repair, but it can't rebuild a sill that's no longer there. Restorers often use a converter on the surrounding steel and inside box sections after welding, because those are the places rust comes back first.

Frequently asked questions

Can I put rust converter straight onto the rust?
Yes, once you've brushed off the loose, flaky rust and degreased. The converter needs rust to react with, so don't sand it all back to bright metal.

How long before I can paint over it?
With Rust Halt, after 3 hours. It converts in about 20 minutes, but give it time to dry fully before you prime or paint.

Will rust converter hide rust from an MOT tester?
No, and it shouldn't. Testers check the metal underneath, and structural rot has to be repaired. A converter is for treating rust on sound steel.

Can I use it on aluminium or galvanised panels?
No. Rust Halt is for iron and steel only. Keep it off aluminium, stainless, chrome and galvanised surfaces.