Failed the MOT on corrosion: what can be welded and what cannot

Most corrosion failures can be repaired. The rusty metal gets cut out, sound steel goes back in its place by seam weld, and the car passes the retest. What decides whether that is worth doing is not how bad the hole looks but where it sits: corrosion within 30cm of a seat belt anchorage, a suspension mounting or a brake mounting is judged far harder than the same hole in the middle of a floor pan, and rot that has crossed into a second structural area is usually where the sums stop working.

This guide covers what DVSA means by excessive corrosion, how a tester reaches that decision, what you can check with a torch, how the repair is carried out, what moves the price, and when a car has gone too far. We weld structurally in our own bays at Rumney, so this comes off the ramp.

What does excessively corroded mean on an MOT?

Excessively corroded means the metal has been weakened to the point where it can no longer carry the load it was designed to carry. That is the whole test. A panel can be brown, scaly and ugly and still be sound, and a sill can look tidy under fresh underseal while the steel behind it has gone to lace.

The tester works from the DVSA inspection manual under body, chassis and structure, the category behind 3.18% of all class 3 and 4 failures. The judgement is about structural integrity: does a load bearing member still transfer force the way the manufacturer intended. Corrosion that seriously weakens one is a major defect. Corrosion that affects braking or steering, or leaves a seat belt anchorage unable to hold, moves up to dangerous.

Surface rust is a different animal. Oxidation sitting on solid steel, the sort that wire brushes back to clean metal, is an advisory at worst. The moment a screwdriver or the corrosion assessment tool goes through, it stops being surface rust and becomes a structural question.

Owners panic at a scabby rear arch, which is cosmetic, and shrug at a damp inner sill by a seat belt bolt, which is the one that fails.

What is a prescribed area and the 30cm rule?

A prescribed area is the zone within 30cm of any mounting point for a component that keeps you alive: seat belt anchorages, suspension mounting points, steering component mountings and brake mountings. Draw a 30cm circle around each of those bolts, in every direction, and everything inside it is prescribed.

Corrosion inside a prescribed area is judged far more severely than the same corrosion outside it. That is the 30cm rule in practice. A hole 40cm back along a floor pan from the rear suspension mounting might be a minor defect. The same hole 20cm from that mounting is a fail, because the load path from the road, through the spring, into the shell now runs through weakened metal.

On a typical monocoque car the circles sit around:

• Seat belt anchorages at the B pillar base, the sill, the floor and the boot floor.
• Front subframe mountings and strut tops.
• Rear beam or rear subframe mountings and rear spring seats.
• Steering rack mountings, brake pipe brackets and the servo mounting to the bulkhead.

Repairs inside a prescribed area have to be continuous seam welds in steel of a similar thickness and strength to the original. Stitch welds, rivets, glass fibre and body filler are not acceptable there, whatever a cheap quote tells you. We cut out filler-and-mesh work done elsewhere most months, and the metal underneath is always worse than the day it was covered.

How does the tester decide, and can I check it myself?

The tester decides by hand, with a corrosion assessment tool and a thumb. There is no gauge, no ultrasound and no meter. They tap suspect metal with moderate force, press with finger and thumb, and scrape away loose underseal to see what is really there. The tool must not be used hard enough to damage sound metal, so a panel that fails gave way to a firm tap.

You can do a version of this yourself, before the test rather than after. Get the car up safely on axle stands or a lift, take a torch and a small screwdriver, and work through the places that rot first:

• The sills, inner and outer, especially the last 30cm at each end and the jacking points. Plastic covers hide a lot.
• The floor pan under the carpet and the boot floor around the spare wheel well.
• Inner arches, behind the wheel arch liner where mud packs in.
• Chassis legs, the crossmember and the subframe mounting points.
• The bulkhead around the brake servo, and the seat belt mountings at the sill and B pillar.

Press with your thumb. Tap gently. Bubbling paint that crunches, or metal that gives, means get it looked at. A pre-MOT check before the test date puts the car on a ramp and finds this while you still have time to plan, which beats standing in reception with a refusal notice and four days of MOT left.

Is a corroded sill or subframe safe to drive on?

Answer that by the defect grade first. A dangerous defect means do not drive it, full stop, and doing so risks prosecution whether or not your old MOT has expired. A major defect means the vehicle failed, but you may still drive it legally while the previous certificate runs, or take it to a booked test or a garage for repair. Advisories are notes, not restrictions.

Grade is not the same as safety. A rotten sill on a modern monocoque is a crash structure problem: the sill is a main longitudinal beam in a side impact and part of the load path that spreads a frontal impact away from the cabin. Corroded through, it folds rather than resists, and the survival space around your legs shrinks in the milliseconds that decide the outcome. A seat belt anchorage in weakened metal is worse, because the belt only works while the floor it is bolted to holds.

A corroded subframe carries different risk. It locates the suspension arms and, on many cars, the steering rack, so corrosion at a mounting can let a bolt pull through over a pothole on a road like Rover Way.

Our position is straightforward. Weakened sill, still holding, valid MOT, short drive on quiet roads, that is a judgement call and usually fine. A dangerous grade or a subframe mounting you can push a screwdriver through, and our 24-hour recovery is the better call. Corrosion rarely arrives alone, so plan for MOT failure repairs across the rest of the fail sheet at the same time, because brake pipes near rot are usually next.

How is a corrosion fail actually repaired?

The repair starts with removal, not welding. Trim, carpet, seat belt hardware, sill covers, arch liners and loose underseal come off so the true extent is visible, and fuel lines, brake lines and wiring in the area get protected or moved.

Then the metal comes out. We cut back past the visible rot until the tool meets steel that is clean, bright and solid on both faces, usually further than the hole suggests. Cutting back too little is the most common failure in cheap welding work, because the weld then sits on metal that is already half gone and lifts within a couple of winters.

The new steel matches the original gauge, generally around 1mm to 1.5mm for a floor pan or a sill. Mating faces get weld-through primer so the inside of the joint is not left bare. We MIG weld, and inside a prescribed area that means a continuous seam weld rather than a row of tacks, then the welds get dressed back.

Protection is where a repair either lasts or does not. Seam sealer over every joint, then epoxy primer, then colour and lacquer on anything visible, which is where our bodyshop and on-site spray booth earns its place, because the car never leaves the building for paint. Cavity wax goes in through the factory access holes, so the new steel is protected on the face you cannot see. Underseal last, over protected metal, never over rust.

Welded repair or replacement panel: which is right?

It depends on the area, and most cars need a mix of both. Pressed repair sections exist for the common rot spots on popular models, and where one fits properly it is the better option, because it restores the original shape and strength in one piece.

Fabricated plate is right for floor pans, jacking points, boot floors, chassis leg patches and the odd shapes where no panel is sold. A well made plate, cut back to sound metal and seam welded, is a proper repair and DVSA treats it as one.

Full replacement is right when the corrosion has taken most of the original. Outer sills on many cars are spot welded, so the job means drilling out spot welds, fitting the new skin over a repaired inner sill and re-welding, which is why the inner sill dictates the work rather than the outer one you can see.

Some things do not get welded at all. A corroded rear axle beam gets replaced, not patched, because the beam is a spring element and a weld changes how it behaves under load. Suspension arms, hubs and steering components are never welded. Subframes are replaced in nearly all cases. Anyone offering to weld up your rear axle beam is doing you no favours.

What does MOT welding cost and how long is the car in?

Cost follows the area and the access, and the bands are wide because both vary enormously. At the lower end sits a single jacking point or a small floor pan patch, one plate with easy access. In the middle sit inner sill sections, boot floor repairs and chassis leg patches. At the upper end sit full inner and outer sills on both sides, anything under a fuel tank that has to come out first, and mounting repairs that need painting afterwards.

Cutting back moves the price more than any other factor. The rot you can see is the end of the process, not the start of it, and steel corrodes from the inside of a closed section outwards. A quote given over the phone from a fail sheet is a starting point at best. We put the car on a ramp instead, strip enough to see the real edge of the sound metal, photograph it, and quote from that.

A quote worth accepting itemises the stages: strip and access, cut and fabricate, weld, dress and seal, paint, cavity protection, refit, and the retest. One number with no breakdown hides the shortcuts.

A single patch is often in and out the same day. A pair of sills with paint takes several days, because seam sealer, primer, colour and lacquer each need cure time in the spray booth. We give you the realistic day rather than the optimistic one.

When is a rusty car no longer worth saving?

Stop when the repair cost passes what the car is realistically worth to you, and stop sooner when the corrosion has crossed into a second structural area. That is our line, and we say it out loud on the ramp rather than let you discover it invoice by invoice.

The arithmetic is not just repair cost against market value. Structural corrosion spreads along seams and box sections, so a car needing one sill this year usually needs the other sill and the floor next year. Weigh the whole-life cost: this repair, the repair you can already see coming, and the tyres, brake pipes and bushes that tend to be tired on the same vehicle. One localised rot spot with everything else sound is worth saving almost regardless of value. Sills, a chassis leg and a subframe mounting all going at once is the car telling you something.

Two things change that verdict. Sentiment and rarity are real reasons to spend beyond book value, and we do that work happily. Salvage is the other end: a car with sound mechanicals and a rotten shell is sometimes worth more in parts than in repair.

What we will not do is take a deposit on a car we think should be sold. Structural welding is a job we want, and it is still the wrong answer often enough that we say so.

What happens once the welding is finished?

The car goes back for a retest, and we arrange that rather than handing you a set of keys and a problem. Under DVSA’s rules a vehicle repaired and re-presented within ten working days can normally be given a partial retest, which looks only at the items that failed. We manage the booking and the presentation at the testing station where the vehicle was originally examined, so the retest is a fixed point in the plan rather than something you chase.

The welding gets inspected as part of that. A tester will look at whether the repair is continuous where the prescribed area demands it, whether the material suits the job, and whether the structure has been restored rather than covered. A repair that ends on soft metal shows.

Paperwork comes back in a familiar shape. Your fail was recorded on a VT30 refusal notice listing each defect and its grade, and you may hear the older VT32 advisory notice referred to, since advisory items sit alongside the fail record. A pass produces the MOT certificate, and the result appears on the government’s MOT history check service within a day or so, where any future buyer sees both the fail and the repair. That record is a good thing.

We hand back photographs of the metal before, during and after, and the way we handle MOT bookings and retests keeps the dates, the paperwork and the repair in one conversation instead of three.

Why does the rust come back, and can it be stopped?

Rust comes back through a repair for one reason above all others: the inside of the joint was left bare. A weld burns off primer and galvanising either side, and a repair painted only on the face you can see leaves clean, hot-worked steel exposed inside a closed section that fills with condensation every night. In a Cardiff winter, with salt on Newport Road and damp coming off the estuary at Rumney, that does not take long.

Trapped moisture does the rest. The usual culprits are blocked drain holes in sills and doors, mud packed behind a wheel arch liner, underseal sprayed straight over existing rust so it seals water in rather than out, and repair plates lapped over the original metal instead of butted into it, which creates a new seam that wicks water.

Protection that lasts comes in a specific order. Weld-through primer on the mating faces, seam sealer over every joint, epoxy primer and paint outside, cavity wax inside the closed section, and underseal last over metal that is already protected. Cavity wax products such as Dinitrol creep along seams and stay soft, which is what you want, because a hard coating cracks and hides the damage. Clear the drain holes, and clear them again in autumn when leaves come down along the Rhymney levels.

That order is the difference between a repair that outlasts the car and one that fails its third MOT from now.