Car underseal: what is it and is it worth doing?

There was a time when cars were very poorly rustproofed and may have started showing signs of corrosion in the first year of ownership. Fortunately, things are much better these days, but that doesn't mean the threat of rust has gone away completely.

Rust on modern cars can often be hidden under more visible bodywork, and it's a very common cause of MoT failures and cars heading to scrapheap. So what can you do to prevent it from claiming your car? A common remedy is underseal, and there are some other rustproofing measures you can take. Our guide assesses all the options.

Some car brands are more susceptible to rust than others, and you should pay particular attention to secondhand cars imported from Japan – an early Nissan Leaf, perhaps – as cars sold there aren't extensively rustproofed due to a mild climate and roads not being treated with salt in winter.

Generally, underseal or other rustproofing measures can be a sound investment to prolong your car's life and cut down on potential big bills further down the line. It's particularly advised if you live in a coastal location, habitually drive on poor-quality roads or have already seen MoT advisories for corrosion on the car. Even in the latter case, acting now can prevent the problem from getting worse.

Do you need car underseal?

What is underseal exactly? Simply put, it's a tougher kind of paint intended to protect areas of the car you don't normally see, such as the inside of the wheelarches and the underside. It's thick and doesn't look very nice, but it can tolerate being hit with lots of debris and chippings much better than a normal paint finish, as it's slightly flexible.

Underseal is applied when a car is built in the factory, but this initial layer can reduce and become damaged in time. That lets salt and water get at the bare metal underneath. Meanwhile, mud or other moisture can get stuck behind pieces of trim like sill covers. And it's possible to damage the coating while jacking a car up for changing a wheel or tyre.

The inevitable result of this is the formation of rust. Left to spread uncontrolled, this can become too expensive to be worthwhile repairing, in relation to the value of the car. Small patched welds can be enough to get the car through an MoT, but only replacing a panel completely will guarantee it won't return.

Car underseal options

A number of treatment types exist, and they all have their pros and cons, as follows. The least expensive is a kind of bitumen (a material also used in road surfacing). A small container of it will only set you back about £5, and it's a workable solution for an older, cheaper car that you're not expecting to be on the road for much longer anyway.

It doesn't look pretty and is very hard to remove once applied, so make sure you tape off any areas you don't want it sprayed on, and use a mask and gloves while applying it. A thick coat of bitumen can give you a few years' protection, but it will start to break down and crack eventually.

Pricier than bitumen are wax-type underseals. Their chief advantage is that they can 'self-heal' if they're hit by a stone or other debris, so the panel covered continues to be shielded from damage. Like bitumen, though, they can wear off after several years, and frequent pressure-washing of the car's underside can accelerate this process.

At the top of the underseal tree are treatments that incorporate rust-converting chemicals and corrosion-inhibiting compounds. For a car that's showing early signs of corrosion, a substance like this can both treat that and ward off further problems.