Underside of vehicle generally corroded: MOT rust advisories explained

"Underside of vehicle generally corroded" is the tester noting surface rust across the underneath of the car, with nothing bad enough to fail. It's common on older cars, especially ones that have done plenty of winters on salted roads. It doesn't need fixing straight away, but it's the early stage of the rust that does fail cars, so it's worth treating before the next winter.

Rust advisories that name a part, like a sill, floor, subframe or jacking point, are more serious. They mean the tester has seen corrosion on metal that holds the car together.

Surface rust vs structural rust

Surface rust is the orange or brown film on bare or chipped metal. The metal underneath is still solid. Brush it with a wire brush and you'll find firm steel.

Structural rust is when the metal itself has got thinner. It flakes off in layers, bubbles up under paint, or has turned into holes. Push on it with a screwdriver and it gives. On a sill, floor pan or subframe, that's a problem, because those parts carry the weight of the car and hold the suspension and seat belt mountings in place.

Why some areas matter more on the MOT

The MOT is strictest about rust in what the manual calls prescribed areas. Broadly, that's load-bearing structure within about 30cm of the mounting points for things like the steering, suspension, brakes and seat belts. Excessive corrosion that weakens the structure there fails the car.

Rust on a non-structural panel such as a door bottom isn't treated the same way unless it leaves a sharp edge that could injure someone.

The detailed rules are in section 6 of the MOT inspection manual, if you want to read exactly how testers judge it.

Repairs have to be done properly

A rusted structural area has to be repaired by cutting out the bad metal and welding in new steel. Testers look for repairs that aren't up to standard, and filler or fibreglass patched over a hole in load-bearing metal isn't an acceptable fix. It just hides the problem.

Welding prices depend heavily on how much rust there is once the garage starts cutting. It's common to find more than the tester could see. Ask for a price for the visible work and an idea of what happens if they find more.

Treating "generally corroded" before it gets worse

You can slow surface rust down a lot, and doing it on a dry day in late summer or early autumn is ideal.

1. Wash the underside, ideally with a pressure washer, and let it dry fully.
2. Wire-brush any loose rust back to firm metal.
3. Apply a rust converter or rust-inhibiting primer to the bare areas.
4. Spray cavity wax into box sections like sills, through the existing drain holes and grommets.
5. Finish exposed areas with a wax-based underbody protection rather than a hard bitumen coating, which can crack and trap water.

Keep the products off the brakes and the exhaust. And don't coat over brake pipes so thickly that a tester can't see them, or you'll get the "covered in grease or other material" note explained in our brake pipe guide.

Buying a car with rust advisories

Corrosion advisories that get longer each year are a pattern worth noticing when you're buying. Our guide on checking MOT history before buying explains how to spot it.

What to do now

If the advisory is just "generally corroded", treat it this year and keep an eye on it. If it names a sill, floor, subframe or jacking point, get a garage to put the car on a lift and show you what they see. Then paste the rest of your result into the MOT advisory decoder to see what else needs doing.

Related guides

• Brake pipe corroded MOT advisory: what it means and what to do
• Suspension arm bush deteriorated or worn: what the MOT advisory means
• How to check MOT history before buying a used car