# Seam sealer and corrosion at lapped joints

> Where rust actually starts on a repaired panel, why a bead over a wet seam makes it worse, and the two preparation steps that prevent most comebacks.

Updated: 2026-11-10 · Source: https://karmindadhesive.com/resources/seam-sealer-corrosion/

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.