A guide to corrosion in cars

How do cars rust, how bad can it get and what you can do about it

This guide is also available as a video:

What is rust?

Most cars have a body, chassis, and suspension made partly or wholly out of steel. Steel exposed to air oxidises to form ferrous oxide or rust. Usually these parts have a coating or are in some cases galvanised to temporarily prevent this process. However over time this will fail and every steel car part will rust to some extent. The process is accelerated by conditions such as temperature, humidity and exposure to other compounds such as salt which is why in colder, damper climates cars rust much more quickly.

How do cars rust?

The age of a vehicle is much less relevant to the advancement of rust than the quality of the original design and manufacture of the car and the conditions it has been exposed to.

When discussing rust on cars I think it's important to say there are a variety of ways in which cars can be affected by rust. If you have owned an old car you will probably have more experience of this, and if you own a classic car or are interested in becoming the owner of one it's important to be familiar with this topic.

Rust on the body panels of a car that is clearly visible may not itself be a huge issue - not all body panels are structural. However it is a strong indicator of more significant problems lurking underneath or within the structure of the vehicle which can be very expensive to rectify. Holes and cracks that appear in the chassis, suspension and seat belt mounting points compromise the safety of the car. Some rust can be a purely cosmetic problem often referred to as 'surface rust'. But this is a misleading term, as many cars have structures susceptible to rusting from the inside out.

Most buyers' guides for cars will alert you to specific areas of concern for a particular model. But generally there are two key areas that usually rust on most cars. Those are the wheel arches (both around the lip of the arch and inside) and the sills. The sills are the lower sections that run from front to back on both sides of a car underneath the doors. Trapped moisture will accelerate rust where the sills meet the wheel arches, often affecting the rear of the car first. This will eventually spread to other parts of the car including the floorpan, pillars and front and rear bulkhead. Boot floors can also often rust out due to the accumulation of water let in through leaking boot seals or rear lamp gaskets.

Ways of dealing with rust

Regardless of where rust has appeared on a car at some stage it should be remedied. The decision of what to do and when to do it depends on a lot of factors. In my view the most important factors are where the rust has occurred and how advanced it is. Though you won't know the true extent of the problem until the work is started to treat or remove it.

Rust prevention, treatment and removal can be done in a number of ways:

  • Use of anti corrosion coatings (rust inhibitors)

  • Use of rust conversion or removal compounds

  • Abrasion: sanding or grinding out the rusted steel

  • Physically cutting out rusted sections of steel or whole panels and replacing them via welded or otherwise bonded fresh steel

The more advanced the rust is and closer it is located to structural areas, the more likely the only way to deal with it is by cutting and welding. This is because once rust becomes well established the steel is weakened and crumbling away causing holes to appear. At this point you can very easily poke holes in the steel with a screwdriver or even your finger. I've seen plenty of cars with this extent of rust. In isolation it's nothing to worry about but it does need dealing with. There is no method to revive steel once it gets to this point, which is commonly referred to as 'rot'.

However small holes in non-structural areas can be bridged by 'filler' with glass fibres and some small holes can also be plugged by weld without the need for repair panels.

The only way to learn how to assess and deal with rust on cars is with hands on experience. Even specialists who deal regularly deal with rust won't be proficient in all types of corrosion on all vehicles and all methods of repair or restoration. That said an experienced professional who deals with rust repairs or restoration who has inspected your car should be able to advise on the extent of a rust problem and estimate how much it will cost to rectify.

The most difficult part is deciding how much to invest in remedying the rust on a car. Different people take different approaches according to their use of the car and budget. Some might decide to deal with immediate issues as they appear, keeping the car on the road as much as possible. Others may decide to have the whole car stripped and all rust dealt with (typically a significant part of a classic car restoration). Or some might do something in between and regularly apply liberal amounts of rust inhibitor to slow the progress of the corrosion.

The cost of rust repair

Costs for rust repairs depend on the approach taken and extent of the rust. They can be anything from an hours labour in a local garage, to hundreds of hours at a restoration specialist. In the UK this could mean anything from £50 to £10, 000 or more. The thoroughness of the work done means the repair could last 1 year or 15-20 years depending of course on how the car is stored and used.

For example let's say you have hole in the sill of your car about the size of a large coin (say a 50p or £2 in the UK). Once ground away you may find that opens up to a hole the size of an apple. We'll assume the rust doesn't extend to any other panels or the inner structure of the vehicle. A patch can be made up and welded for this relatively inexpensively, but it will still need painting. Usually a much larger area will need to be painted to blend the repair in. A small independent garage or body shop could charge around £150 - £400 for the work. A tiny hole in the floor might only cost you £50 and a significantly rusted whole sill replacement (inner and outer) might cost £700 - £1000.

You can also have rust inhibitors professionally applied to the underbody, wheel arches and sills of your car for around £300 - £500

It's hard to estimate the longevity of a rust repair or treatment but if you are spending more than a few hundred pounds it would be reasonable to expect the repair to last for several years and for those doing the work to guarantee their workmanship for a period of time.

You can obviously save money by doing the work yourself as it is a labour intensive process and the cost of rust treatment products and repair panels are typically fairly low. Many rust repairs are within the realms of DIY but it does require a degree of skill so don't expect your first attempts to be anywhere near the quality of a professional. Many people, myself included, have taught themselves how to weld (perhaps with a little help) in order to repair or restore their own cars. There are different welding processes but MIG welding is regarded as the most versatile and straightforward for work on cars. But extensive rust is very hard to repair on your own without access to the knowledge, tools and experience of someone who has done it hundreds of times.

How to inspect a car for rust

Rust isn't something to panic about if you have just noticed it on your car. But it is something to be mindful of. Particularly if you're buying a used car over about 7 years old, or a classic car that hasn't been restored within a similar time frame. Even if a car is relatively new or you are looking at a classic that supposedly had any rust dealt with you should still look out for it.

To fully inspect a car for rust it needs to be raised for your inspection or photos taken of the underside. Alternatively if you're able to, get on the floor and inspect the sills or underbody with a torch. Feel for rot along the seams and look for holes and bubbling of paintwork. Potentially even ask if you can pull back the carpeting inside the car to inspect the inner sills and floorpan. Check the strut mounting points under the bonnet (in the engine bay). Look for clues that might add up to a bigger problem lurking. It's entirely possible that even an honest seller may not have discovered rust before on their car that has been there for a while. After all how often do you really thoroughly inspect your own car yourself?

In the UK you can also check the annual MOT test history to determine if the car has had problems with corrosion and subsequent repairs, however bear in mind this only provides snapshots of the history of the car over time and a failure can be the result of a very small amount of rust in a potentially dangerous location, likewise an 'advisory' can be the result of a much bigger problem and vice versa.

If you want to understand how much a car is affected by rust you need to gain an overall picture of the health of the steel components that make up the car. Then you can make an informed decision about it.

Finally be wary of cheap rust repairs of your car or a car you are looking to buy. It is hard to determine the quality of a repair once it is covered in an anti corrosion wax or filler. Both are valid coatings to apply over a repaired area but any repair to the structure of the car should be properly 'seam' welded all the way round. The only way to determine this is with photographic evidence of the work. Even paint can hide the signs of the quality of welding. A simple trick to check for solid steel is to use a magnet. But generally, as with the extent of rust, the quality of a repair is something only experience can help you to determine. Even MOT testers have to make a subjective judgement call on work carried out and decide whether it meets certain criteria. That said it is much rarer in modern times that people go to the effort of badly repairing a car. This is why on classic cars you might see repairs involving rivets, adhesive and even concrete and chicken wire, whereas to find such a repair on a modern car would be highly unusual.

I hope you've enjoyed my exploration of rust in cars and found it useful. Personally I find dealing with rust on cars satisfying and working with steel fascinating. If you'd like more detail on anything covered in here, let me know.