Welding repairs for rust and MOT failures: what to expect

By RPM Autocare, Newton Abbot · Published 26 October 2026

Rust is a fact of life for cars, particularly in a damp, coastal climate like South Devon's. It becomes a real problem when it eats through structural metal, and an MOT tester can fail a car for it. If your car has been flagged for corrosion, or you have spotted bubbling paint, this guide explains what is going on and how a welding repair works. It is general information, and the right repair always depends on the car.

Why rust matters

Rust spreads. Once it has bubbled through the paint, the metal underneath is usually worse than it looks. Surface rust on a panel is mostly cosmetic. But rust in structural areas, those that carry load or hold parts such as suspension in place, affects strength and safety.

Where cars usually rust

• Sills and jacking points.
• Wheel arches and rear quarters.
• Door bottoms and boot lids.
• Floor pans and around suspension mounts.

Some of these are easy to see. Others are on the underside, which is why the problem often only becomes obvious when a car is on a ramp at test time.

How the MOT treats corrosion

An MOT tester inspects the structure and the areas around key mounting points. Where corrosion weakens a load-bearing area, the car can fail. Lesser corrosion may be noted as an advisory, which means it is worth keeping an eye on, or dealing with before it gets worse.

We do not carry out the MOT test ourselves. It is done at a test station, and we can arrange one alongside a service if you ask. What we can do is deal with the repair after a corrosion failure or advisory. Ask the test station about retest rules, as these depend on the circumstances.

Why filler is not the answer

It is tempting to fill a rusty area and paint over it. But filler over rust does not last, because the rust keeps spreading underneath. Where the metal has gone, it needs cutting out and replacing. Filler might look fine for a short while, and it may not pass a tester's inspection of a structural area either.

How a welding repair works

The approach at RPM Autocare is:

1. Look at the car. Mark inspects the corrosion, shows you how far the rust goes and explains what is involved.
2. Cut out the rotten metal. The damaged section is removed back to sound metal.
3. Weld in new steel. A new piece is shaped, fitted and welded in.
4. Seal, prime and paint. The repair is sealed and painted in our spray booth to match the rest of the car, so it is protected and lasts.

The repair is priced on how much metal needs replacing, which is hard to know until we have looked. It is priced on the work involved plus materials. For a rough idea, the bodywork estimator has a rust and welding option, and you will get a proper quote once Mark has seen photos or the car.

Is it always worth repairing?

Not always, and an honest garage should tell you either way. On a car of modest value with extensive corrosion, the cost of repair can approach or exceed what the car is worth. On a car you want to keep, or where the rust is localised, a proper repair can be sensible. The best thing to do is to get the corrosion looked at and make the decision with the facts.

What to do if your car has failed on rust

1. Get the failure sheet from the test station and keep it.
2. Check whether the car can be driven. If a defect is marked dangerous, do not drive it. Ask the test station how to proceed.
3. Send photos of the areas to Mark, or bring the car in, so he can see how far the rust goes.
4. Ask about mechanical work at the same time. If the failure includes other items, such as brakes, Russ can sort those too.
5. Check the retest rules with the test station before the car goes back.

If rust has been flagged on an MOT, Mark can do the welding and Russ can sort any mechanical work, all under one roof. That saves moving the car between businesses.

Catching rust early

• Wash the underside after salted roads, where practical.
• Fix stone chips and scratches before rust takes hold.
• Look at sills, arches and door bottoms regularly.
• Do not ignore bubbling paint. It is usually the visible end of a larger problem.

Early repairs are generally smaller and simpler than waiting for a hole. A quick tidy of a chip, or a panel repair, is less work than welding in new metal.

What about a check before the test?

If your car is getting on a bit, or you know rust is lurking somewhere, ask for a look underneath before the MOT so there are no surprises. Our guide to why cars fail the MOT on brakes covers another area worth checking in advance.

Get it looked at

See our welding page for more detail, or our bodywork and paint page for the wider range of work Mark does. Bodywork start dates depend on what is in the booth, so send Mark a few photos for a quote and he will give you a realistic date. You can contact us or book in. We are based in Newton Abbot and welcome customers from Kingsteignton, Kingskerswell, Teignmouth and Torbay.

Related services: Welding · Bodywork & paint · Bodywork estimator · Repairs