Car underseal rust protection: what is it and is it worth getting for your car?

Protecting a new, used or classic car from rust and corrosion with a rust-proofing car underseal is something many owners will have considered. Rust and corrosion is a serious problem for cars of all ages, with vulnerable areas like the chassis and wheel arches being constantly exposed to dirt, salt and moisture. However, you can protect these parts by applying a car underseal or cavity wax. This page outlines the different types of underseals available, how to apply them and the likely costs involved.

Look in any scrapyard and you will see a surprising number of relatively modern vehicles which have succumbed to the MoT tester’s screwdriver. Water and moisture can easily get behind plastic trim and bumpers, with rust eating away at the bodywork and chassis long before it’s noticed by a mechanic or the owner, and can easily become uneconomical to repair if left untreated.

From small city cars to high-end luxury sports cars, applying a car underseal is a good way of preventing and delaying rust and corrosion from appearing. It should help keep your car roadworthy for longer and is more cost effective than welding repairs further down the line. 

Imported cars, especially those from Japan, may not be extensively rust proofed, as the climate is much milder and the roads are not treated with corrosive salt in the wintertime so owners of these vehicles need to take particular care. Those who live by the sea are also up against harsher, saltier conditions and extra rust proofing can be advisable.

What is a car underseal?

A car underseal is a tough rust protection paint or wax designed to repel water and dirt from exposed areas of a car that are likely to be exposed to the harshest road conditions, such as the chassis and wheel arches. Applying an underseal is relatively easy and can be done from your driveway. There’s a range of treatments to suit most budgets.

Car underseal and waxes

There are several different types of cavity waxes and underseals, each having different costs and benefits.

Bitumen

The cheapest form of underseal is simply a type of bitumen – the same material which is used to hold roads together. It costs around £5 for a small can and is effective at protecting ‘banger’ type cars which you don’t expect to last long term. It doesn’t look pretty though, and make sure you wear gloves and mask off any areas you don’t want to be coated, as it is tricky to remove once applied. Normally applied thickly, this type of underseal provides decent protection for about a year, but over time this will crack and will allow water, dirt and moisture to meet the metal again, encouraging rust – so regular reapplication is a must. 

Wax underseals

Wax underseals are more expensive, but have the ability to ‘self-heal’ if hit by road debris, which means the panel is always protected. After a few years, though, it can wear away - especially if the underside of the car is regularly pressure washed. In really hot weather it can even melt, leaving some areas with thinner protection. The best wax underseals will combine corrosion-inhibiting and rust converting chemicals in the coating to prevent further corrosion.

Treating inside box sections like chassis rails and sills is also a good idea, although a different type of chemical can be used for this, or underseal products can be made thinner with a white spirit to inject into inaccessible places. Wax underseals are typically more expensive than bitumen, but for around £80, it’s definitely cheaper to apply this regularly than having welding repairs later on.

How to apply underseal 

Thankfully, treating a car with an underseal is something which can easily be done on the driveway at home by a keen DIYer. Be warned though, it can be a messy job so wearing eye protection is a must and you’ll also need decent access to the underside of the car via a ramp or axle stands.

To do the job properly you’ll need to make sure the surface to be painted is clean and that the old, cracked underseal is removed – this can be done using a wire brush. If there are any rust holes it’s best to get these repaired, as covering it will only make the welding job more difficult (or impossible) later.

You can either use a paintbrush, aerosol or spray gun to apply the seal. Make sure you mask off areas which shouldn’t be covered – especially the brakes. Protect the exhaust too – although it’s not disastrous if it is coated, the heat will burn the underseal but it will smell unpleasant. It’s also worth covering the paint on panels you will see, as overspray is difficult to remove.

Access to the sills and other enclosed areas can usually be gained by carefully removing a blanking plate or grommets. If there are wheel arch liners or other bits of trim, it is worth dropping them down to access the metal behind. When underneath the car, make sure you spray from different directions to ensure even coverage.

Once your car is treated, you can reapply any trim pieces and can relax knowing you have protection. Remember to thoroughly check for rust regularly though as treating rot at the earliest opportunity is cheaper in the long run. You can use a rust converter on any areas of surface rust, as well as reapplying the underseal every few years to keep on top of rot.

Alternatively, if you aren’t able to do it yourself, then some garages, bodyshops or specialists will be able to do it for you – expect it to cost between £350-£500. Be sure to go on recommendations though.

Can I underseal my car if it already has rust?

New car owners will know if their car has received any additional rust protection since leaving the factory, but buyers of secondhand or used cars might not be so lucky. Thankfully, rust protection can be applied even if your car has sections of surface rust, but you will need to brush away any surface rust with a wire brush and apply a rust converter beforehand. Any rust which has eaten through bodywork or other metal surfaces may require attention from a bodyshop before a sealant can be applied.

Has your car ever succumbed to rust? Let us know in the comments below...