Car Rust Repair: How to Spot, Treat and Prevent Rust

Rust is one of the few kinds of car damage that gets worse on its own. A chip that reaches bare steel starts corroding the moment it meets damp air, and by the time you can see a brown blister on the paint, the affected area underneath is already larger than the mark on the surface. Deal with it early and it is a small cosmetic job. Leave it and it becomes cutting, welding and, on a structural area, an MOT failure.

What is rust and why is it a problem for cars?

Rust is iron oxide, the reddish brown compound that forms when iron or steel meets oxygen and moisture. It is a chemical reaction that consumes the metal, so rusted steel has literally lost material rather than simply changed colour.

That is why it matters more on a car than on, say, a garden gate. Three consequences follow:

• Structural strength drops. Sills, floorpans, chassis rails and suspension mounting points carry load and are designed to behave predictably in a collision. Steel that has corroded through cannot do either.
• Components fail. Brake pipes, fuel lines, exhaust mountings and suspension components all corrode, and when they fail they fail suddenly.
• Value falls. Visible corrosion is one of the first things a buyer or a trade valuer looks for, because it suggests problems they cannot see.

Rust also spreads. Once it is established under a paint edge, moisture wicks along beneath the coating and lifts it, so the patch you can see is the smallest the problem will ever be.

What causes rust on cars in the UK?

Moisture and road salt, in that order, and Britain supplies both in quantity. Rain, damp air, morning dew and standing water all put moisture against the metal. Winter gritting then coats the underside of every car with salt, and salt water is a far more aggressive electrolyte than plain water, so corrosion accelerates sharply between about November and March.

Other contributors:

• Stone chips and unrepaired scratches, which break the paint system and expose bare steel directly
• Kerbing and minor knocks that crack paint on sills and wheel arch lips
• Blocked drain holes in doors, sills and boot lids, which trap water inside the panel where it cannot dry
• Coastal and estuary air, which carries salt all year round
• Trapped mud in wheel arches and behind arch liners, which holds moisture against the metal for weeks
• Neglected washing, especially over winter, which leaves salt sitting on the paintwork and underside
• Poor previous repairs, where filler has been applied over untreated corrosion

Cars that sit unused for long periods are not exempt. A car parked on grass or in a damp, unventilated garage often fares worse than one driven daily, because it never gets warm enough to dry out.

Where does rust usually start on a car?

It starts where water collects and where the paint has been broken. If you are inspecting a car, these are the places to look first:

• Sills and the seams along the bottom of the doors
• Wheel arch lips, especially the rear arches
• The bottom edges of doors, where drain holes block
• Jacking points and the floorpan around them
• Around the fuel filler flap
• The boot floor and spare wheel well
• Around the windscreen and rear screen surrounds
• Bonnet and boot lid edges, where the skin is folded over the frame
• Suspension turrets, subframes and brake pipe runs
• Behind wheel arch liners and under plastic trim

Every one of those is either a water trap, a folded seam or a place where stones and debris strike the paint.

Does rust fail an MOT?

Corrosion in a prescribed structural area fails the MOT, but cosmetic rust on an outer panel does not. The distinction is about function, not appearance.

A tester will fail a car for corrosion that is:

• Within 30cm of a mounting point for the suspension, steering or seatbelt anchorages, where the metal is significantly weakened
• Serious enough to have holed a load bearing member such as a sill, chassis rail or floor
• Present on a brake pipe to the point where it is excessively corroded or leaking
• Holing the floor or bulkhead so that exhaust fumes can enter the cabin

The tester checks by hand as well as by eye, tapping and pressing suspect areas, so filler and underseal sprayed over rot will not get a car through. A rusty wing or a corroded boot lid edge, by contrast, is a cosmetic matter and does not affect the result, although it is still worth fixing before it spreads to somewhere that does.

How do you identify rust on your car?

Look for orange or brown staining, blistering paint and metal that feels rough or pitted rather than smooth. Rust is easy to spot once you know what you are looking at.

The visual signs, in the order they usually appear:

• Staining. A brown or orange mark, often running downwards from a chip or seam.
• Bubbling paint. Small raised blisters. Rust expands as it forms, and it pushes the paint up from underneath. Bubbling paint is always worse than it looks, because the corrosion has spread out under the coating.
• Flaking and peeling. The paint has lost its grip on the metal entirely.
• Pitting. The bare metal surface is rough and pockmarked instead of smooth.
• Holes. The end stage, where the metal has been consumed.

Wash the car before inspecting it. Road film hides early staining, and a clean panel in daylight shows up bubbling that is invisible on a dirty one.

How to check hidden areas for rust

Most serious corrosion is out of sight, so check underneath and inside as well as on the outside. A few minutes with a torch will tell you more than a walk round ever will.

• Use a torch. Good light is the single biggest help when looking under a car or into the boot well.
• Use an inspection mirror. A small mirror on a handle lets you see round the back of sills, arch lips and inner wings.
• Press and prod. Push firmly on suspect areas with a screwdriver handle or your thumb. Sound steel is rigid. Rotten steel flexes, crunches or gives way.
• Lift the boot carpet. Check the spare wheel well, which is a classic water trap on cars with a leaking boot seal or blocked drain.
• Check inside the doors. Look at the drain holes along the bottom edge and clear them if they are blocked with dirt.
• Look behind arch liners. Where they can be pulled back safely, there is often trapped mud sitting against the metal.
• Watch for damp carpets. A wet footwell nearly always means water is getting in somewhere, and the floor beneath it is corroding.

If you find soft metal rather than surface staining, stop poking and get it inspected. There is no point patching a hole if the surrounding metal is equally far gone.

How bad is it? Surface rust, scale and rot

Corrosion goes through three stages, and each needs a different repair. Identifying which you are looking at determines whether this is an afternoon's work or a structural job.

The practical message is that surface rust caught early is the cheap version of this job, and every month it is left moves it closer to the expensive version.

Do some cars rust more than others?

Yes, and it comes down to the metal and the coatings used. Materials differ in how readily they corrode. Stainless steel contains chromium, which forms a protective barrier against oxygen and moisture and makes it far more resistant than ordinary mild steel. Aluminium panels, common on bonnets, boot lids and wings of newer cars, do not rust in the red iron oxide sense at all, though they do corrode in their own way, producing a white powdery oxide that lifts paint.

Coatings matter just as much:

• Galvanised steel carries a zinc layer that corrodes in preference to the steel beneath it, which is why galvanised bodyshells last so much longer
• Cavity wax injected into sills, doors and box sections protects the inside of panels where paint cannot reach
• Underseal and stone chip coatings protect the areas taking the worst of the road debris
• Rustproofing treatments add an oil or wax barrier over vulnerable areas

No car is immune. Given enough time, moisture and salt, even well protected steel corrodes eventually. Protection buys years, not permanence, and it only works where it is intact. A stone chip through a galvanised panel's paint will still rust at the chip.

How do you prevent rust on your car?

Keep it clean, keep it dry and repair damaged paint promptly. Those three habits prevent the great majority of avoidable corrosion.

Wash regularly, especially over winter

Washing removes the salt, grit and dirt that hold moisture against the paint. Through the winter, wash more often rather than less, and pay attention to the underside, wheel arches and sills. Many jet wash bays have an underbody lance, and using it after a spell of gritted roads is worth more than any product you can buy.

Wax or seal the paintwork

Wax or a synthetic sealant puts a barrier between the paint and the elements, and helps water bead and run off rather than sit. It also makes the next wash easier. It is not a substitute for repairing damage, but on sound paint it slows everything down. A machine polish and protect once or twice a year keeps the coating in good order.

Repair chips and scratches quickly

This is the one that actually stops rust rather than delaying it. Every stone chip that reaches bare metal is a corrosion site. Having stone chips repaired while they are still just chips is one of the cheapest pieces of preventative maintenance there is.

Use rust converters and inhibitors correctly

Rust converter reacts with existing surface rust to form a stable compound, and rust inhibitor sprays put a barrier between metal and air. Applied by brush, roller or aerosol, they are inexpensive and useful in the right place, typically on underbody and inner panel areas that are never going to be painted to a cosmetic standard.

Their limits are worth being clear about. A converter is not a repair. It stabilises light surface corrosion on metal that is otherwise sound. Applying it over flaking scale, or over a hole, achieves nothing. Follow the manufacturer's instructions on surface preparation and safety, since these are chemically active products.

Keep the car dry

Storing the car in a dry garage or under a car port helps, provided the space is ventilated. A sealed, damp garage can be worse than parking outside, because the car never dries out. Clear the drain holes in doors, sills and the sunroof so water can escape, and deal with any leak that is wetting a carpet.

How is rust repaired on a car?

The method depends on how far the corrosion has gone, and the one rule that applies to all of them is that the rust must be removed, not covered.

Sanding and refinishing

For surface rust, the corrosion is sanded or ground back to clean bright metal, the metal is treated, then primed, painted and lacquered. It is straightforward work and suits small patches on outer panels, such as staining around a chip or a scratch that has started to go. Anyone with basic DIY skill can tackle a small area, though the paint match and finish will not be to body shop standard without proper equipment.

Cutting out and patching

Where the metal has gone soft or holed, the affected section is cut out entirely and a patch of new steel let in. This suits localised rot in a panel that is otherwise sound, such as a wheel arch lip or a small area of floor. The patch must be properly seam sealed and protected on both sides, or it will corrode from behind within a few years. A patch is not appropriate on a load bearing structural member.

Welding

Welding is used where corrosion has damaged structural sections: sills, chassis rails, suspension mounting areas or floor sections. New metal is welded in, seam sealed and protected. This requires the right equipment and genuine skill, and on any load bearing area it should be done by a qualified professional. Done badly, it is worse than not doing it, because it looks repaired without being strong.

Priming and painting

Whatever comes before it, the repair is finished by sealing the metal from the air again: etch or epoxy primer on bare steel, then colour and lacquer on outer panels, or stone chip coating and cavity wax on the underside and inside of box sections. Miss this stage and the repair simply starts rusting again.

When should you get a professional to do it?

Get a professional involved when the corrosion is structural, when it is spreading faster than you can keep up with, or when you want the finish to be invisible. A body shop can assess how far the rust has actually gone, which is nearly always further than the visible area suggests, and tell you honestly whether the car is worth the work.

Signature has been repairing bodywork from the unit at Radcliffe since 2008 and offers a full vehicle body repair service, with a mobile service across Greater Manchester, Lancashire, Cheshire, Merseyside and West Yorkshire for the cosmetic work that can be done on site. Quotes are free and answered within 24 hours on 0161 667 0919 or at enquiries@signatureawbr.co.uk, and all parts and workmanship are guaranteed.