Stop rust before it starts – hands-on reviews of 2025’s best car underseal

FAQs on car underseal

• How does car underseal work?

  As well as wheels and subsequent need for wheel sealant, the underside of your car is subjected to a lot of wear and tear. From stones flying underneath it, chipping away at coatings to salt, water and other contaminants. It can all quickly become a breeding ground for rust. As the underbody is crucial to your car's structural rigidity, failing to stop rust can cost you a fortune.

• Is car underseal important?

  The answer is an immediate yes. Important structural parts of a car will be inspected during an MOT. Rust can see an immediate MOT failure, so it does make sense to check cars for rust and prevent it from forming. Underseal works as a thick layer of protection. It ensures that rocks and stones bounce off and that salt and other debris doesn't begin rusting your car. The whole process is quite a bit more thorough than, say, applying black trim restorer.
  
  Happily, however, we're far from the days where cars barely lasted a decade before they were ready for the scrapyard. Factory underseal and rust protection has improved massively in recent times. However, keeping such things in good running order still takes effort, which is why underseal is a worthwhile purchase. Things like tyre dressings are more of a treat-yourself item in comparison. And even if you are running a classic in all weather, it's a no-brainer if you want to keep your car on the road.

• How often should I apply underseal to a vehicle?

  If you drive a more modern vehicle, it's a good idea to check the manufacturer's rust guarantee. Because if the factory-applied underseal is still doing its job properly, the best thing you can do is simply keep the underside clean with a hose or pressure washer.
  
  If however, you're in an older vehicle, the general consensus from most underseal brands is to reapply a layer of underseal on a yearly basis - ideally just before winter. If the underside needs welding beforehand though, get that sorted before applying anything, as underseal won't stop developing corrosion.

• What is car underseal?

  Underseal is a protective coating for the underside of your car. It can be used on the floor pans, chassis rails, subframes and other bits that are exposed to the dirt, grit and salt from the roads.
  
  It's designed to prevent corrosion from developing. Rust is a big killer of cars as they get older, so if applied to a car before rust develops, treating it to an undercoating can help make them last for longer. Equally, you should really underseal your car after a restoration to ensure it lasts for as long as possible.

• How to apply car underseal?

  If you want to apply car underseal properly, you're going to need a tool that can elevate your car high enough into the air so you can reach al the tricky areas. If you've got access to a lift, great. Otherwise, you could manage if you use some jacks in a safe and prepared manner.
  
  When applying the stuff, you could begin with the little holes and crevices, so you can protect the most difficult-to-access areas first. Places like the holes for the jacking points and running the thin tube throughout the chassis rail and making sure you've got good coverage.
  
  Then, carefully, you move onto the outer areas, ensuring you're getting enough coverage to not leave any bare spots. Each product will vary in volume and longevity, so it's a case of you keeping on top of maintaining the underside as and when you need to.
  
  You need to ensure you're protected yourself. A good set of goggles and protective gear is always a good idea when applying underseal. Like paint, it's quite toxic stuff, so you don't want to be breathing in any of the residue - or even have it come into contact with your skin. So, be well-prepped before going ahead.