Believe it or not, but applying car underseal is one of the best things you can do to ensure the safety of your pride and joy. Keeping a car shiny from the best method of paint protection is all well and good, but the underside doesn’t have a nice coat of paint to protect it from the elements.
No matter how well a coating of car wax works on the bodywork, it’s underneath where you’ll find the nasties. Take a peek underneath any car a few decades ago, and you’ll either see the worrying signs of rust forming or a thick black coating of primitive underseal, often made of bitumen.
Older cars have their fans for various reasons, but rust protection was never one of them, and steps need to be taken to avoid expensive repairs. You can make a car look gorgeous with a ceramic coating, but it’s a whole other story underneath. Thankfully, modern underseal is affordable and easy to apply, and we’ve been hard at work finding the best examples on sale. We’ve even tested an example for ourselves.
The best car underseal 2025 at a glance
If you do happen to own an older car, or you just want a more thorough protection than a dash of quick detailer. Aftermarket car underseal (or underbody protection) is a hugely effective way to keep your car safe from the elements.
The best car underseal 2025
It went through a fairly thorough test over the winter and early spring months. I applied a portion of it to my Mercedes 230E in December 2023, and after looking underneath as of May 2024 (five months), the metal is still very strong despite occasional use on salty roads. Of course, I ensured all the salt had gone by giving the underside and wheel arches a rinse after each journey. But the underseal did its job well and protected the underside from loosening up and turning crusty. The real test however, is how it'll cope over the next winter.
Review by Aaron Hussain
Pros
- Very easy to apply and aim into the general spots
- Provides very decent protection
- Very efficient in its spray
Cons
- Length of bottle can make it tricky to aim for certain areas (if you're spraying while on the ground)
- Does have a strong smell
Best underseal bundle
This three-pack set is the best value, in our opinion. It's a shame you only get 500ml bottles to play with, but nevertheless, each use should provide ample protection to the underside of your car. And the great thing is that it can be applied to other objects and surfaces as well. Be aware that it's not a clear coat and that the aerosol sprays out black.
Pros
- Comes in a pack of three
- OEM approved for both classic and modern cars
- Can be applied to a number of surfaces
Cons
- The bottles are only 500ml each
- It's a black spray, so be careful where you apply it
Best for after a refurb
Unlike the Hammerite, you'll need to remove existing rust, and the surface must also be free of dust, dirt, wax, or grease before applying the spray in order to get the best out of it. This can be a bit of a fuss before coating your car, but the protective qualities are excellent.
Pros
- Will treat your underside very well
- Clever tech and easy to apply by spraying on
- Ideal for both painted and unpainted surfaces
Cons
- Existing rust needs to be removed first
- Surfaces must be free of dust, wax, and grease before use
Best value rust protection
The coating protects the undersides from salt and rocks, and the inclusion of Waxoyl will help keep rust at bay while also sealing in existing corrosion, stopping it spreading.
Pros
- Comes in large tin
- Can easily be applied with a paintbrush
- Provides a thick layer of pretection
Cons
- Not as straightforward as an aerosol
- Doesn't look as neat after application
Best for a number of applications
It can be used on clean or previously rusted metals and is a solid choice if you don't want the underside of your car coated in a black layer. If you want to preserve a rusted look on an old vehicle or object (i.e. like a Rat Rod project), this is the ideal spray to protect the metal.
Pros
- Ideal for a number of surfaces
- Easy to apply
- Great for a rat rod-style project
Cons
- Others offer more thorough protection
The best value underseal
It's designed to be protective against rust and stone chips, durable, and to be applied to the underbody and on sills. The Under Guard also comes in a generous can of 500ml, which for the price it's offered at, makes it very nice value for money compared with the other products, which can easily stray over the £15 or even £20 mark.
Pros
- Great value for money compared to the others
- Heat resistant up to 120°C
- Generous 500ml quantity
Cons
- Protection for black paint, so be careful where you apply it
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.
Ryan Gilmore is the Deputy Autos and Tools Editor for Parkers, specialising in car cleaning and hand tools. A veteran car cleaner; if it details, waxes or washes he’ll be on hand to make sure it’s worth your money.
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