Vehicle rust can become expensive and potentially dangerous if left untreated. If you notice rust on your car, don’t hesitate before treating it.
New Zealand is an island surrounded by salt water and blown by strong winds, so it’s no surprise that items made of metal will rust relatively quickly here. Motor vehicles are particularly vulnerable and, while you might think rust only affects older cars, newer cars are also susceptible to underbody corrosion, especially if you live in a coastal suburb.
Rust develops when iron reacts with oxygen and water to produce oxidation that appears as a reddish-brown coating. This, in turn, can lead to corrosion. Rust forms in stages and understanding where a rust spot is in its decomposition process can help point you to the right solution. If left untreated, it can grow and spread quickly.
Here are the three main stages of rust and how you can deal with them.
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Surface rust
Most surface rust on cars forms when paint breaks down through mechanical or UV damage, which is why even cars in warmer, dryer climates get it. As vehicle paint ages, it becomes vulnerable to wear and abrasions and, once water penetrates the unprotected metal, it oxidises. Pure iron, stainless steel, and aluminium don’t oxidise as aggressively, but normal steel tends to contain impurities in the metal that accelerate the rusting process.
Surface rust only affects the top layer of your vehicle’s body panels. This rust appears in chips and scratches in the protective paint coating, or as bubbles under the paint surface that have started on the surface of the steel panel under the paint finish.
While surface rust won’t cause major damage to your vehicle right away, it’s best to treat it as soon as you notice it. Part of a vehicle Warrant of Fitness check includes structural condition and, in some cases, rust can result in your vehicle failing its WOF.
You can fix small chips and scratches by using a small artist’s brush with a good automotive rust converter or rust inhibitor. This eats away the oxidation and stops the chemical rusting process. You should then apply a paint undercoat and finish treatment with a top coat. You can then touch up the area by polishing it to a high gloss to match the rest of the vehicle’s paint finish.
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Scale rust
If you don’t remedy surface rust and allow decomposition to penetrate further into the metal, you may see bubbles start to form in your car’s paint. This is because iron oxide molecules are bigger than those of iron or steel. By expanding and flaking away the outer layers of paint and metal, this scale rust exposes the base metal, which then corrodes. Eventually, scale rust will work its way through the surface of the body panel and affect the integrity of the metal itself.
Treating scale rust requires more aggressive techniques and the method required depends on the vehicle type, the condition of the vehicle, and the location of the scale rust. Options include:
- Grinding
- Sanding
- Media blasting
- Dry sandblasting
- Wet sandblasting
- Acid dip stripping.
If there are signs of surface bubbles in the affected areas, it will need to be sanded down or stripped back to bare steel. Make sure you get down to the bare metal, so any sealants or treatments applied can stick to it.
Once this is done, you’ll need to treat the steel surface with a phosphoric acid-based product as this will kill the surface rust and leave a phosphate film on the steel surface. The next steps in the process are to coat the bare steel with an etch primer, fill and finish the surface, then apply an undercoat and top coat paint finish.
If the scale rust has developed further within the steel, they may need to be repaired or replaced.
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Penetrating rust
If left unchecked, rust will eventually eat through the vehicle’s metal and create holes, leading to more dangerous problems like a weakened vehicle frame and suspension components. If you live in an area where you’re surrounded by salt and rust-prone conditions, it is recommended you don’t wait until bubbles appear on your car’s body. Routinely check underneath your vehicle to inspect for any rough or compromised parts that could be a safety risk on the road if left unattended.
You shouldn’t repair large holes with body filler as you would with pits left by scale rust. You have two options: you can completely replace the affected part or panel or cut the degraded parts out and weld metal patch panels into place. Either way, both of these options are difficult to DIY, and you should get the job inspected then repaired by a reputable vehicle body repairer.