industrial · selection
Sealing container and trailer seams: STP or PU
Updated 2026-11-10
A trailer is a mixed-material assembly: galvanised steel frame, aluminium or GRP panels, plywood or composite floor, powder-coated fittings. The sealant has to bond to all of them and stay flexible at motorway speed.
Two seams, two answers
| Seam | Product | Why |
|---|---|---|
| Exterior roof and panel laps | STP (KAM921) | UV stable without topcoat, primerless on most substrates, paintable |
| Interior floor and structural seams | PU seam sealer (KA540) | Sandable, takes topcoat, 600% elongation |
Using PU on the roof means the seam needs a topcoat to survive UV; using STP on the floor means a soft bead where abrasion resistance matters.
Movement on the road
A trailer body flexes as it loads, unloads and travels. The bead sees cyclic movement at the panel laps and vibration at the frame joints. A sealant with high modulus and low elongation cracks at the lap; a softer bead follows the movement but wears faster under abrasion.
Surface preparation on galvanised steel
Galvanised surfaces carry oil and a zinc passivation layer. Neither is a bonding surface. Degrease, abrade lightly, and test a coupon; if the bead peels cleanly, use the activator or a primer before the production run.
A seam map for a typical trailer
| Seam | Substrate | Movement | Product |
|---|---|---|---|
| Roof lap | Aluminium or GRP to frame | Thermal, wind | STP |
| Side panel joint | Coated steel / GRP | Flex, vibration | STP |
| Floor to wall | Plywood / steel | Load, vibration | PU sealer |
| Rear frame | Steel | High vibration | PU sealer |
| Door seal | Mixed | Abrasion | Profile, not sealant |
Mapping the seams before ordering prevents the most common purchasing error on a trailer line: one product bought for every joint, then either a soft bead on the floor or a UV-sensitive bead on the roof.
Why the roof seam leaks first
The roof sees the largest temperature swing and the most direct sun, and it is the seam nobody inspects.
aluminium panel 2.4 m, 60 °C surface swing
2,400 mm × 60 °C × 23 × 10⁻⁶ = 3.3 mm of movementThat is 3.3 mm of expansion against a bead bonded to a steel frame that moves by a different amount. A PU bead with 600% elongation can absorb it; a thin, hard bead cannot, and it cracks at the lap where the two materials meet.
Three details make the difference:
- A bead with section. A roof lap sealed with a wiped-thin smear has no material to stretch.
- Continuity over the crown. A bead that stops at a rivet line and restarts leaves a capillary gap; water travels along the lap and appears a metre away.
- UV stability, or a topcoat. PU without a topcoat chalks and cracks in two or three summers; STP does not.
The seam most often missed: the floor-to-wall junction
Water thrown up by the wheels enters at the floor-to-wall seam, not at the roof. It is usually sealed with a PU sealer because it takes abrasion and paint, and it is usually the first seam to fail because the panel flexes under load and the bead is applied too thin.
On a trailer line, this seam deserves a specified bead section and a cure window, not a pass with whatever is left in the gun at the end of the shift.
What this means for buyers
- Specify STP for exterior laps, PU for interior structural seams.
- Test adhesion on the actual galvanised and coated substrates, not on bare steel.
- Budget for UV: if you choose PU on the roof, the topcoat is part of the system.
- Buy 600 ml sausages for line work; cartridges are for repair.