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The best underseal to protect your car from rust

  • Rust can be a car killer
  • While newer cars have better rust-proofing, more protection is welcome
  • Find the best underseal here to keep your chassis protected 

Written by Ryan Gilmore Updated: 1 May 2024

Believe it or not, but treating your car to the best underseal is one of the best things you can do to ensure its safety. Keeping a car shiny from the best ways 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 underseal 2024 at a glance:

Editor’s pick: Bilt Hamber Dynax UC – Buy from Amazon.
Best for after a refurb: Liqui Moly Bitumen Underbody Protection – Buy from Amazon.
Best value underseal: Simoniz Black Under Guard – Buy from eBay.

If you do happen to own an older car, or you just want a more thorough protection than a dash of quick detailer. Aftermarket underseal (or underbody protection) is a hugely effective way to keep your car safe from the elements.

The best underseal 2024

Our top choice for underbody protection, Bilt Hamber Dynax UC, is a clever, see-through wax that offers top-notch protection without changing how your car looks. The wax is durable and 'self-healing' to ensure that you get full coverage, and the fact it's an aerosol means it's really easy to apply, even to awkward areas. I personally found it easy to aim and target certain areas, but the length of the 750ml bottle made it tricky to aim in some cases, as I was doing my best while lying on the ground (not recommended - get the car elevated) and it did smell quite strong. But in short, I can only report that it did the job - and very well for that matter. As a precaution, you should really be wearing a mask, gloves, and safety goggles before applying any underseal.

Bilt Hamber Dynax UC

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
Not only is Dinitrol's underseal wax OEM approved and ready for use with both classic and modern car undersides, but it's super easy to apply, and they will often sell it to you in packs.

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
A classic choice of underseal with a modern twist, Liqui Moly Bitumen Underbody Protection makes use of bitumen to seal and protect the underbody of your car but is sprayed on for easier application.

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
The old faithful of rust protection, Hammerite Underseal is as basic as it comes. It comes in a one-litre tin and can be applied with a paintbrush for a rough but useful layer of protection. It's designed to be applied in the wheel arches and throughout the underbody on major chassis rails and flooring.

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
Price: $14.99
A clear coating for rust removal, Jenolite Rust Shield is a tough layer that can be applied to all metal surfaces to give excellent protection against rust. This not only extends to cars and motorcycles but garden furniture and ornaments as well.

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
Offering exceptional value for money and heat resistant up to 120°C, the Under Guard protective paint by Simoniz is a great budget option in the world of underseal. It's designed to be as simple as it can be, whereby you simply spray onto the desired areas without fuss.

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

  • Is protection black paint, so be careful where you apply it

FAQs on underseal

How does it 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 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 it 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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