Corrosion in a structural area

What causes it

• Road salt and standing water, particularly where the drain holes in sills and box sections have silted up
• Stone chips and kerb scrapes breaking the paint film and letting water reach bare steel
• Poor previous repairs — filler, glass fibre or plated patches laid over rust that was never cut out
• Water leaks from a perished screen seal, a blocked sunroof drain or a failed boot seal, rotting the floor from the inside out

Advisory or failure?

Where the corrosion is matters far more than how much of it there is. Any area within 30cm of a seatbelt anchorage, a suspension or steering mounting, a brake component mounting or a body-to-chassis mounting is a prescribed area, and corrosion there that has weakened the structure — judged by the tester using finger and thumb pressure or a corrosion assessment tool — is a major defect and a failure. Where the weakening is severe enough to affect the vehicle's structural integrity, the grading is dangerous. Outside the prescribed areas the bar is lower: a holed floor or sill that does not weaken the structure may be recorded as a minor defect or an advisory, though corroded bodywork with sharp edges likely to cause injury is a major. The repair standard is strict — within a prescribed area the repair must be continuously seam welded and at least as strong as the original, and filler, glass fibre, pop-riveted plates and a coat of underseal over the top are not acceptable.

Can you fix it yourself?

Garage job

Welding in a prescribed area is structural work: it needs a MIG welder, the skill to lay a continuous seam in thin and often rotten metal, and the judgement to keep cutting back until you reach sound steel. It also has to be done clear of the fuel tank, fuel lines and brake pipes. Cosmetic rust on a wing or a door is a reasonable home project, and keeping sill drain holes clear and washing salt off the underside through winter genuinely slows the whole problem down — but the moment the rot is anywhere near a mounting point, it belongs with somebody who welds for a living.

What it costs

A single localised patch — a sill section or a small floor repair — is typically £250 to £500, and nearly all of that is labour, since the steel itself costs very little. Cost scales with how much has to be cut out, how much has to be removed to reach it, and whether the fuel tank, exhaust or interior trim must come out first. Both sills, or a chassis rail and outrigger on a vehicle with a separate chassis, reaches the top of the band and past it. Get the car properly assessed before committing: a good welder will tell you what they expect to find once they start cutting, and on a car worth £1,500 the honest answer is sometimes not to start at all.

What happens if you ignore it

Corrosion never stops on its own; it works outwards from inside a box section, so what you can see is always the smallest part of it. In a prescribed area the consequence is that a seatbelt anchorage or a suspension mounting tears out under exactly the load it exists to carry — a crash, or a heavy pothole at speed. Financially this is the defect most likely to end a car's life: a repair that would have been £300 last year is £1,200 this year, and at some point it exceeds what the car is worth, which is how most older cars in Britain finally leave the road.

Other common defects

• Brake disc worn, pitted or scored
• Brake pads wearing thin
• Braking efficiency below requirement
• Tyre worn close to the legal limit
• See all defects →