Rust converters and rust repair: Parkers’ ‘how to’ guide

We look at the problems caused by rust on cars, how to avoid it, and how to treat it.

Rust around a vehicle's wheel arch

by Chris Williams |

Rusting is an unfortunate but common problem with cars. Here we delve into why rust is a problem and the best ways you can go about avoiding and fixing it yourself.

What causes rust?

This occurs when iron is subject to oxidation, that is, it reacts with oxygen to create iron oxide, commonly known as rust.

It’s simply a chemical reaction. Fe (iron) reacts with oxygen (O) to create Fe2O3 (iron oxide).

Cars are not made from pure iron anymore, the last time you would have seen a new cast iron engine block would have been some decades ago. These days, carmakers use steel (iron with a small percentage of carbon) alloys, aluminium, magnesium, titanium, and even carbon fibre.

You’re unlikely to see some of those on your everyday Volkswagen because they are very expensive materials reserved for high-end cars. Even aluminium is much more expensive than steel and is used frugally.

That leaves us with steel and steel alloys. These are cheaper than the other materials but also subject to oxidation at much faster rates. In order to rectify this, steel in cars get protective coatings or alloyed to fight against corrosion. It helps a lot but it doesn’t completely solve the issue. Protective coatings wear off and rust on vehicles still occurs. And the situation is not helped by Britain’s salted roads. In a nutshell, salt speeds up the rusting process.

Common rust areas on cars

Rust will often manifest itself in a number of common areas around a car:

Wheel arches, suspension, around the windscreen, around doors, under the boot carpet, along the frame rails and sills, and on the exhaust.

You may see rust on steel brake discs when a car has been sitting for a while but you needn’t be concerned about this. It’ll disappear once the car is used and the rotors have been turned a few times.

If rust is occurring beneath paintwork, bubbling paint is a normal indication of this. Air and moisture have seeped in. See below.

Paint bubbling around a car door handle
©Photo: Getty Images

Rust prevention

Ideally, you want to try and avoid rust in the first place. And there are a number of products that will help you here. One of these is likely to be on your garage shelf already: WD-40. The WD stands for water displacement and it is fantastic for use in small or tight areas of your car where moisture and water can gather, such as hinges.

It’s also very important you keep your car clean and dry. Grime, salt, and contaminants that gather on your car (especially in the wheel arches and underside) during driving aid corrosion and should be removed frequently. To give your car exterior a thorough clean, see our guide on the subject. But in short, the essentials are some car shampoo, some paint protectant, and a drying cloth.

You may also wish to consider an underseal to apply to areas such as sills. This forms a barrier to help protect against corrosion.

Dents and scratches that penetrate a car’s paintwork are certainly worth fixing. Quite apart from it being unsightly, the exposed metal can soon rust.

Rust converters and rust products

Has rust already appeared? Not to worry, it can be fixed in most cases.

There is quite a bewildering range of rust products that is it all too easy to become confounded. The first thing you ought to do is determine what type of rust it is that is plaguing your car. But they all bear a common factor: you absolutely must follow application instructions to the letter otherwise it’s a waste of time.

Surface rust is the initial stage and the easiest to cure. Nipping rust in the bud at this stage is your best option.

To cure surface rust you still need to strip away paint if there is any and also the rust. The bare metal must be clean and degreased. Once it is you can apply the relevant product. We recommend the Bilt Hamber Hydrate 80 because it is exceptional at converting the rust, killing the spread, and protecting against any subsequent rusting.

Hydrate 80 is applied in two coats with a brush and must be left for 24 hours before priming and painting over the top.

Scale rust is the next stage. The aforementioned bubbling of paintwork is often a sign of scale rust beneath. If you wish to treat yourself, you must strip away scaly and flaky rust. Because it is more severe than surface rust, this will take a little more effort. Like surface rust, you can then apply a remedy – we suggest Hydrate 80 once again.

Penetrating rust is when rust has advanced right through the metal. This is a major problem for several reasons, including structural rigidity. The simplest option at this point is to get the part or panel replaced.

Stripping rust

The fastest way of stripping back paint and rust is with an angle grinder fitted with a wire brush or abrasive flapper wheel. You can also get wire brushes for power drills too. By hand, you can use heavy or medium grit sandpaper.

Read next:

Tyre sealant: Parkers' guide

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The best car tool kits for home mechanics

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