The short answer
Fixing car body rust in the UK ranges from a low three-figure sum to treat and respray a small patch of surface rust, up to several hundred or several thousand pounds where the corrosion has eaten through the metal and a section has to be cut out and welded. The deciding factor is how far the rust has gone. Light surface rust can be sanded back, treated, primed and resprayed. Once rust has perforated the metal, the affected section must be cut out and a new piece welded in before painting — far more labour. Rust on or near structural and MOT-critical areas like sills, subframe mounts and suspension points is the most serious and expensive, and can fail an MOT.
Rust repair cost depends entirely on whether the metal is still solid or has rotted through. The sections below explain the stages of corrosion and what each needs.
Rust repair at a glance
- Small surface rust patchLow three figures
- Perforated section, weldedSeveral hundred pounds+
- Structural / sill repairHigh hundreds to thousands
- Key factorSurface vs through the metal
- MOTStructural rust can fail
Surface rust versus rot through the metal
The cost of a rust repair is set by how deep the corrosion has gone. Surface rust — the early orange staining or a small bubble under the paint — sits on top of metal that is still solid. It can be sanded or wire-brushed back to clean steel, treated with a rust converter or etch primer, then primed and resprayed. Caught early, this is a relatively cheap panel repair.
Once rust has gone right through the metal, leaving holes or paper-thin steel, it cannot just be painted over — filler over rusty metal is a temporary cover that rusts straight back through. The proper repair is to cut out the corroded section and weld in new metal, then seal, prime and respray. That is skilled labour and far dearer. The worst case is widespread rot, where multiple sections need replacing, at which point the repair cost can approach what the car is worth.
| Rust stage | Repair | Typical UK range |
|---|---|---|
| Surface rust / small bubble | Sand, treat, prime, respray | Low three figures |
| Localised perforation | Cut out, weld patch, paint | Several hundred pounds+ |
| Sill / structural section | Cut and weld new section | High hundreds to thousands |
| Widespread rot | Multiple sections / often uneconomic | Thousands |
Indicative UK ranges for guidance only; cost depends on extent, location and access.
Rust and the MOT
Rust is not just cosmetic — it affects roadworthiness and the MOT test. Corrosion is assessed by where it is and how bad it is. Surface rust on a wing or door is cosmetic and will not fail an MOT. But corrosion within a prescribed area — typically within 30cm of a structural or load-bearing component such as suspension mounts, steering, seatbelt anchorages or the brake lines — is taken seriously, and significant corrosion there is an MOT failure.
Sills, subframes, chassis rails and suspension turrets are the parts that matter most, because they carry load and keep the car safe. Rust that has weakened these is both a safety issue and a test failure, and repairs there must be done properly by welding in sound metal — not filled or patched cosmetically. This is why an advisory for corrosion on an MOT is worth acting on before it progresses from an advisory to a failure, and why an early surface-rust repair is far cheaper than leaving it to eat into structure.
Where rust starts and why it spreads
Knowing where rust tends to begin helps explain why early treatment is so much cheaper. Corrosion usually starts where water and road salt collect or where the paint has been broken: wheel arches, sills, the bottoms of doors, the boot floor and around the fuel filler are classic spots, along with anywhere a stone chip or scratch has exposed bare metal. Once the paint seal is broken, moisture reaches the steel and rust begins, often spreading underneath paint that still looks intact — which is why a small surface bubble can hide a larger patch of corrosion beneath.
Hidden areas are the ones that catch owners out. Rust frequently develops on the inside of panels, in box sections and behind trim, where it cannot be seen until it bubbles through to the surface or weakens the metal. Drain holes in doors and sills that have become blocked trap water and accelerate the process. This is why a repairer will often probe and inspect areas around visible rust rather than just treating the spot you can see — the corrosion you can see is sometimes only the edge of what is happening underneath, and pricing the job properly means establishing how far it has actually spread.
When rust repair is and isn't worth it
Whether to repair rust depends on the car's value and how far the corrosion has spread. On a newer or valuable car, or a classic worth preserving, catching rust early and repairing it properly is sensible — it protects both safety and value, and a small surface repair now avoids a structural one later. Treating stone chips and small bubbles before they spread is the least costly rust management of all.
On an older, lower-value car with significant rot — rusty sills, floors or structural sections — the welding cost can easily exceed what the car is worth, at which point repair stops being economic. A good bodyshop or MOT tester will tell you honestly how far the corrosion has gone, often by prodding suspect areas to see whether the metal is still solid. For structural rust, only a proper cut-and-weld repair by a competent welder is acceptable; cosmetic patching of load-bearing areas is unsafe and will not pass an MOT. Prompt action on surface rust is always the least costly route, because corrosion only ever gets worse and more expensive the longer it is left.
Frequently asked questions
Can rust just be painted or filled over?
Only light surface rust on solid metal can be sanded back, treated and resprayed. Rust that has gone through the metal cannot be filled over, because the corrosion continues underneath and comes back through. Perforated or rotten metal must be cut out and replaced with welded-in new steel for a durable, safe repair.
Will rust fail an MOT?
Cosmetic surface rust on a wing or door will not fail an MOT, but significant corrosion within a prescribed area near structural or load-bearing parts — such as suspension mounts, sills, brake lines or seatbelt anchorages — is an MOT failure. Corrosion advisories are worth acting on before they worsen into a failure.
Is it worth repairing rust on an old car?
It depends on the extent and the car's value. Catching surface rust early and treating it is cheap and worthwhile on any car. But once corrosion has rotted sills, floors or structural sections, the welding cost can exceed an older car's value, at which point repair may not be economic. A bodyshop can advise how far the rust has spread.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific car and damage. They are guidance, not a quotation.