Seam Sealer
Quick answer: Seam sealer is the flexible bead run along a car's welded seams at the factory and painted over. The welds hold the car together but are not watertight on their own; the sealer is what keeps the water out. It dries, shrinks and cracks with age, and then the seam leaks. Not to be confused with a paint sealant, which is a protection product for the paintwork.
A car body is assembled from pressed panels joined by rows of spot welds. As the write-up on a Mazda 3 puts it, spot welding is fine for holding the car together but is not waterproof, so a sealer is added over the top. It is then painted, which is why most owners have never knowingly seen it.
What it means
Seam sealer is a flexible body sealant, applied as a bead along the joints between panels: roof gutter strips, tailgate and boot apertures, floor pans, sills and the closing seams of box sections. It has to stay flexible, because the shell flexes constantly as the car is driven. The paint on top ages at a different rate to the sealer underneath, so a seam can look perfectly sound and still be open.
Why it matters
It is a consumable that nobody consumes. Left alone for ten or fifteen years it dries out, shrinks and cracks, and a cracked seam is a direct path into the car, usually into a cavity where the water is not seen until it has gone somewhere else. The Lexus RX400h write-up describes exactly that around the tailgate hinge seams: sealed at the factory, painted over, and dried out and cracked with age.
The important reassurance, from our Ford Focus write-up, is that a leaking seam is not a structural fault: the welds are doing their job, it is only the waterproofing over them that has failed. Or, as that page puts it, there is no danger your car is becoming unwelded.
How it is put right
The repair is unglamorous and durable. The old sealer is chased or chiselled out, the seam cleaned back, primed, and a fresh bead run along it. On a Ford Transit Connect the leaking seam was inside a rear box section and had to be found with a camera behind the panels before any of that could start, which is the usual difficulty: reaching the seam costs more effort than sealing it.
Common mistakes
- Reading a leaking seam as a structural problem. The welds are sound; it is the sealing bead over them that has aged out.
- Running fresh sealant over the old cracked bead without chasing it out first, so the new material bonds to failing sealer rather than to metal.
- Judging a seam by its paint. The paint can look perfect over a bead that has split underneath it.
- Assuming a car that has had accident repair was resealed to factory standard. Seams disturbed in a repair are a regular source of later leaks.