Factory seam sealing and why it must be reproduced

Open your boot or look inside a door opening and check the joints between panels. You will see beads of a rubbery material, sometimes smooth, sometimes with a distinctive texture or pattern, painted the same colour as the body. That is the factory seam sealing. It goes unnoticed, but it does important work, and reproducing it properly is one of the signs of a careful repair.

What seam sealing does

• Keeps water out. Where two panels overlap there is a narrow gap. Unsealed, water and moisture get in and stay there, and that is where a lot of corrosion starts.
• Prevents noise and vibration. Sealed seams do not rub or rattle against each other, making the car quieter.
• Blocks dust and draughts. Well-sealed joints stop dust, sand and air getting into the cabin or boot.
• Strengthens joints. Some body sealers and adhesives also add stiffness to certain joints, such as the hems of outer door skins.

Why it gets lost in a repair

When a welded panel is replaced, the original sealing goes with it. And when an area near a seam is repaired, the sealer often has to be removed to work on it. If it is not replaced at the end, the joint is left open. The car leaves the workshop looking good, but with a way in for water that eventually becomes rust.

Reproduce it, don't just cover it

Any old bead of silicone will not do. A proper body seam sealer must bond to the metal, stay flexible through heat and cold, accept paint and not crack over the years. The repair also aims to make the result look original:

• The same type of bead on each seam: flat, rounded, brushed or sprayed, depending on what the factory did.
• The same texture. On many cars the sealer has a rough or brush-marked finish, which is imitated with the right tool.
• In the same place. Not too much, which would look like a blob, and not too little, which would leave gaps.

This matters for a practical reason too: anyone inspecting a car, for example before buying it, tends to look at the sealing. A bead that is uneven or different from the other side of the car gives a repair away. Well-reproduced sealing does not draw attention.

How it's done

First the seam is cleaned thoroughly, removing old sealer, grease and silicone. Bare metal is then protected with an anti-corrosion primer. The sealer is applied to the seam with the appropriate method, shaped and textured, and left to cure. Finally it is painted along with the rest of the panel so that it blends in and matches the colour.

On the underside and sills, sealing is complemented by stone-chip coating, which protects those areas from flying stones. And inside closed sections, at the end of the process, cavity wax is applied to complete the protection from within.

An unsealed seam doesn't show on collection day. It shows when the rust appears.

Bonded glass

Bonded glass, such as the windscreen or rear window, is a similar case. It is fixed with an adhesive that seals and also contributes to the stiffness of the body. If it is removed during a repair, it must be bonded back in with suitable products and allowed the curing time they require before the car is moved.

If you want a repair that looks after these details too, see our accident repair and rust protection treatment pages. And if you are buying a used car, our pre-purchase inspection checks the sealing among many other things.