Introduction to underseal

The condition of the underside of a vehicle is frequently ignored, on older cars or vans this can lead to severe rust development. By selecting the best car or van underseal you can extend the life of the vehicle considerably.

best car underseal
Hammerite Underseal is a firm favourite for use in the UK

In this guide we will look at the best products to use to underseal a car or van and also the recommended stages that are required when undersealing.

Top picks


  • Manufacturer - Hammerite
  • 2.5L recommended for a complete car or small van
  • Paint on with a brush
  • Thick protection that contains waxoyl
  • Excellent corrosion resistance

  • Manufacturer - Schutz underseal
  • Roughly 5 to 6 cans needed for car or small van
  • Can be brushed, rollered or sprayed on using a compressor
  • Best applied on a warmer day. Takes roughly 2 hours to touch dry
  • Highly flammable so take care when applying

  • Manufacturer - Liqui Molly
  • Roughly 4 to 5 cans needed to complete a car or small van
  • Application - Spray on
  • Thixotropic and is therefore applied as a non-drip coating
  • Surfaces which have to be treated with underseal black must be thoroughly cleaned beforehand.

It is extremely common for the underside of early VW Transporters (T4’s and prior), Transits and Bongos to require a high level of maintenance due to relatively poor factory finishes during the 1990’s to early 2000’s.

If you are buying a second hand Bongo or VW T4 it is worth double checking that this maintenance has been carried out.

Bongos especially suffered from a lack of adquate production underseal, the better Bongo examples therefore should have been undersealed at the point of import to ensure that they could cope with UK road conditions.

VW T4 bodywork needing best underseal for car or van
This rust developed on a 2001 VW T4, the T5 and T6 have much better production underseal

Rust around critical mounting points is one of the most common causes for MOT failure.

Historically most vehicles showed signs of rust by the time they were due for their first service. Over time the factory finish on the chasis/underside of cars improved and in the early 00’s this technology, later this was also introduced to vans.

Finding the best underseal

The most popular underseal products used in the UK for both cars and vans are:

  • Liqui Moly Underseal Bitumen, Black 500ml (View here)
  • Hammerite Underbody Seal (View here)
  • Hammerite Underbody Seal Aerosol With Waxoyl 600ml (View here)
  • Car Body Schutz Black Underseal 1 litre Textured Underbody Coating (View here)
  • Schutz Underseal Black Heavy Duty 1 Litre (View here)
  • Rejel Dinitrol 4941 Black Underbody Wax (View here)
In the process of undersealing a VW T4

How the best underseal developed

As underseal technology has improved in line with the life expectation of new vehicles, the factory finish on vans now includes a robust protective layer of underseal to the chasis, floor pans, mounting points, sills, inner arches etc.

The best underseal technology is now used during production and does extend the life of the underbody significantly, although does eventually chip away. Older vehicles (over 12-15 years old) therefore need the underseal maintaining.

High quality factory finish using the best underseal technology
A high quality factory finish

Despite being undersealed during production, modern vehicles, still need regular monitoring and rust-proofing action is recommended to keep them safe from silently dissolving away because of rust.  

What is underseal

What then is undersealing? The best underseal is a tough paint formulated to offer protection to the bottom of the car or van, inside the wheel arches and other parts that are not normally seen but are exposed to salty road conditions.

Underseal is applied to the underbody and other hidden van parts (in a procedure dubbed undersealing), to keep the bare metal from coming into contact with moisture, air and salt, which is a critical factor in the redox reaction that supports rust development.

Undersealing a van, whether new or used is beneficial because it prolongs the vehicle’s life and reduces issues being flagged during vehicle inspections / MOT’s.

The best underseal for a car or van

Many products are available for use in undersealing are easily accessible. Some can be obtained off of Amazon, and here, we have sampled out a few.

Schutz Underseal Heavy Duty

Schutz underseal is one of the top-rated products for undersealing tasks. It is famed for providing high impact protective coating to the van’s underbody.

It is ideal for protecting the underbody from salt, water, and corrosion.

Hammerite Car Underseal

best car underseal

The Hammerite underbody seal is one of the UK’s premium brand that does a perfect undersealing job. This product protects against stones, water, and salt.

The waxoyl system has potent rust inhibitors that are perfect in displacing moisture, thus keeping the underbody safe from corrosion. The protections offered by this product guarantees your vehicle a longer shelf-life.

Liqui Moly Underseal Bitumen

Liqui moly is a great spray on underseal

The Liqui Molly underseal offers a shield against corrosion by creating a tough, protective film on the underbody.

This German-made formulation is composed of bitumen, solvents, synthetic resins, and other inhibitors that make it a powerful rust-proofing agent for the underbody.

Rejel Dinitrol Black Underbody Wax

best car underseal

This product is sold as a three in one package. This product’s formulation creates a protective film that shields against water, salt, or corrosion.

How much does it cost to underseal a car or van?

Applying rust-proofing treatment to your van’s underbody is a fairly low skilled task which involves spraying or painting the underside of your vehicle in a sticky tar like paint, it can be a very messy task that is hard to wash off clothes and skin!

Undersealing can therefore be done as a DIY project, or you can find a professional to do the job.

Many different brands offer DIY kits, which are worth considering for a one off project. Underseal is sold as spray on or paint on. Spray on underseal is quicker but is generally more expensive.

How to underseal a bongo

The main cost to complete a DIY job is that of the cans of underseal that you will use depending on the size of your vehicle. A DIY job will set you back roughly £30-£40 in materials (Max). This will give you enough for rust-proofing the underbody of a small to medium sized van such as a Transporter or Bongo.

There is the option for using a professional to rust-proof your van’s underbody, and the cost for this will obviously be higher.

Professionals will use specialized tools to facilitate the rust-proofing process and will usually have access to a hydraulic lift to raise the vehicle. This will also usually use a compressor with spray gun to apply the product.

With better access it allows them to coat even the toughest cavities. A decent job done by a professional would require you to spend anything from £200 up to £500 depending on the amount of preparation work needed.

How long does it take to underseal a car or van?

To underseal our VW Transporter T4 it took approximately 3 hours. It is slightly quicker if you buy spray on products however I find the result is not as good as the hand painted applications.

Whilst car underseal that is applied by paint brush takes a little longer, it is thicker, thorough, cheaper and less messy!

How long do the best car underseals last?

If done right, a rust-proofing job for a van’s underbody can last about 8-10 years. A quality rust-proofing job that stays intact close to a decade will give you the peace of mind that your vehicle is well protected despite the weather conditions.

The scientific explanation to car undersealing

Underseal is a thick tar like formulation that drys solid to create an air tight, water tight seal around metal. The best underseals for use on cars and vans can be painted or sprayed onto the target surface.

Underseal contains chemicals that include rust inhibitors, whose role is to slow down oxidation, an essential factor in rust development.

using the best car underseal
Underseal should be used after repair work to underside of vehicles.

The formulation also contains synthetic resins and bitumen, which when they combine, they create a rigid protective film that protects the bare metal from abrasion.

How to underseal a car or van

Different steps go into the process of undersealing a car or van however they are all very simple DIY tasks if you are physically able.

The different stages are outlined below:

Paint all exposed metal, avoiding the exhaust and brake lines, suspension
  1. Raise the car or van onto ramps or axel stands. If you have access to a pt… Even better
  2. Put on your goggles, you do not want rust flakes or underseal in your eye. Trust me.
  3. Gloves are also advisable as underseal is extremely hard to get off your skin.
  4. Old clothes are also advised as even thick Hammerite drips occasionally!
  5. Clean the car or van thoroughly to eliminate as much loose dirt, mud, grease or oil on the underside as possible. Cleaning should also entail removing loose rust flakes using a wire brush. Working in sections should help.
  6. Rust treatment follows. Here, the goal is to neutralize and prevent the rust from spreading. Rust converters will come in handy here because it serves as a primer sealant. (View rust converters)
  7. Do not paint over damaged metal that requires replacement as the underseal could cover up serious structural issues.
  8. Apply the underseal to all areas of exposed metal but take care to exclude the exhaust system, suspension components and brakes. Avoid the brake lines if possible also.
  9. Finish off with coating the underbody floor pans, Try not to get it onto heater surfaces such as exhausts or engine casings.
  10. Allow the underseal to dry.
  11. You can repeat this process several times to give thicker protection depending on your time/budget.

The Do’s and Don’ts for Van Undersealing

  1. Consider using products that combine rust inhibitors and rust converters, especially if your van’s underbody has started showing rusting signs. This will come in handy to treat your underside and offer protection against future rust issues.
  2.  Consider applying a thick film, which will come in handy to provide years of protection.
  3. If you intend to use bitumen for your undersealing job, ensure you mask off the parts you do not want to be coated as it is very tough/impossible to remove once applied.
  4. If you opt for the DIY undersealing option, do not commence the work without having the right set of protective gear such as overalls, masks, and eye protection.  
  5. Ensure there is adequate ventilation when performing undersealing. Some products have less fumes than others. Hammerite for example does not have an especially strong smell however Schutz is very odorous.


Car and van undersealing is an essential procedure for both new and used vehicles to increase their life expectancy. Selecting the best underseal for a car or van will save you time and money in the longer run. The cost for fabrication repairs is something to avoid for as long as possible

As technology has improved significantly over the years in the reliability of automotive components, it is often the body work that leads to their ultimate demise.

Regular monitoring of your van’s underbody will give hints as to when it would be necessary to perform an undersealing procedure. Van undersealing can be done as a DIY project at home, which is cheaper.

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