Seam sealer and corrosion at lapped joints

Corrosion at a lapped seam rarely starts where the sealer is. It starts at the edge of the bead, in the capillary gap the sealer did not reach, or under a bead applied over contamination.

The mechanism

A lap joint creates a narrow gap between two steel surfaces. Water enters by capillary action, oxygen is available at the edge, and the gap holds moisture far longer than a flat panel. Sealing the outside without treating the gap traps the conditions that cause rust.

The correct sequence is: clean, treat the metal, prime, then seal. Sealer is the last step, not the first.

Where repairs go wrong

• Sealing over rust. The bead bonds to oxide, which is not a stable substrate; the rust continues underneath.
• Sealing a damp seam. Moisture-curing PU will cure, but the bond interface holds water.
• Leaving a gap at the bead end. A bead that stops short of the seam end leaves an entry point.
• Skipping the primer. Some sealers bond to bare steel; the paint system above them may not.

What good looks like

1. Strip to clean metal, remove all oxide.
2. Apply the specified primer or conversion coating.
3. Seal with a continuous bead that extends past the joint end.
4. Topcoat, including the bead edge.

Three corrosion cells, three fixes

Corrosion at a seam is not one process. The repair has to match the cell.

Cell	Where it forms	What stops it
Crevice	Inside the lap, between two panels	Sealing the gap completely, or a cavity wax
Galvanic	Steel against aluminium or a coated fastener	Isolation, or a primer that breaks the couple
Filiform	Under a coating, spreading from a scratch	Full removal of the coating and the oxide before resealing

A bead over a crevice that is already wet makes the crevice worse: it stops the water from drying out. This is why “seal it and see” is the wrong repair for a bubbling seam — the rust continues under the bead and reappears at the edge a season later.

Cavity wax is part of the repair

A lapped seam has two sides. The outside gets the bead; the inside — a door shell, a sill, a wheel arch — needs a cavity wax or an internal coating, because the condensation forms on the inside face.

A repair that seals only the outside leaves the inner face unprotected and the lap still able to corrode from within. On a visible seam this is invisible until it perforates.

Why it returns at the bead edge

The bead edge is where three things meet: the sealant, the coating and the steel. Water sits in the meniscus, oxygen is available, and any pinhole in the coating starts a cell that runs along the interface.

Two details reduce it:

• Extend the bead past the seam end, onto clean painted metal, so the edge is not at the joint.
• Topcoat over the bead edge, sealing the transition rather than leaving a step for water to sit against.

Neither fixes a seam that was sealed wet. They only slow the return.

What this means for buyers

1. Buy sealer as part of a system with the primer and topcoat, not as a standalone.
2. Inspect the seam end — that is where the water gets in.
3. Do not use sealer to cover rust; it hides the problem for one season.
4. Ask the paint supplier which primer goes under the sealer on bare steel.