Fiberglass and Delamination Repair

How a laminated sidewall is put together

The wall on most modern coaches and trailers is a sandwich panel, assembled flat on a table and pressed as one piece. Outer layer: a thin fiberglass skin, usually a rolled filon sheet or a gelcoated laminate. Behind it: block foam, either expanded or extruded polystyrene, cut to fit around an aluminum stud cage. Behind that: a luan plywood layer, then the interior finish. The adhesive holding those layers together is the structure. Remove it and the wall has almost no stiffness of its own.

That construction is why these panels are light, flat, and strong in the plane of the wall, and also why they cannot be repaired like sheet metal. There is nothing to pull a dent out of. Body filler applied over a separated area sits on a layer that is no longer attached to anything, so it cracks in a straight line along the edge of the repair within a season or two.

Older units and many high end coaches use a different build: hand laid or sprayed fiberglass over a framed structure, with gelcoat as the finish rather than paint. Those fail differently. Instead of bond line release you get gelcoat crazing, print through of the framing behind it, and osmotic blistering where moisture has worked into the laminate itself. The diagnosis is different and so is the repair.

Why the bond line lets go

Water is the usual culprit, and it almost never enters through the middle of a panel. It comes in at a penetration: a window flange whose putty tape has hardened and shrunk, a marker light screw, an awning rail, a slide seal, or the seam where the front cap meets the sidewall. Once inside, it wicks along the luan, softens the adhesive, and separates the foam from the skin. The bubble appears three feet away from where the leak actually is.

Heat drives the second mechanism. A dark sidewall in an inland Southern California summer reaches surface temperatures well above ambient, the skin expands, the substrate behind it expands less, and the adhesive works in shear every single day. Panels that were bonded with marginal pressure at the factory, or that got a thin adhesive spread, eventually fatigue at the weakest patch. This is why bubbles so often show up on the sun facing side of a unit and not the shaded side.

Impact contributes too, and it is the most misread cause. A branch strike or a low speed contact that leaves a scuff no bigger than a hand can shear the bond line across two or three feet of wall. The skin springs back and looks nearly normal. Press on it and it moves under your palm, or tap along it and the sound changes from a solid tick to a hollow drum. That is separation, and paint over it will crack.

What re-bonding actually involves

The first job is mapping, not cutting. We tap the panel systematically and mark the boundary of the hollow area, then take moisture readings across it and into the surrounding wall. That tells us whether we are dealing with a dry mechanical separation or a wet one, and whether the luan and the stud cage behind it are still sound. Repairing a wet laminate without drying it first traps the moisture and produces a callback within a year.

Small dry separations can be resin injected. We drill a pattern of small ports along the perimeter and into the field, work a low viscosity epoxy into the void, then clamp the panel flat with cauls and vacuum bagging until it cures. Done properly the result is stiffer than the factory bond, and the only surface evidence is a set of filled ports that disappear under refinish. Done casually, with the wrong resin or without clamping, it simply fills a cavity and hardens into a rattle.

Larger or wet areas mean opening the wall. The outer skin comes off in a controlled cut, the saturated foam and delaminated luan are removed back to dry material, corroded studs are replaced, and the core is rebuilt with fresh block foam and new substrate. The skin is then re-bonded with a structural adhesive, pressed flat, and the seam is glassed and faired. It is more labor and it is the only durable answer once water has been in the wall.

Gelcoat, cracks, and matching the finish

Gelcoat is a pigmented polyester surface layer, not paint, and it ages by chalking rather than peeling. Spider cracking radiating from window corners and cap seams is a stress signature: the laminate behind it is flexing more than the brittle surface layer can follow. Grinding out the crack and refilling it is only useful once the flex is addressed, otherwise the same pattern returns in the same place with the same geometry.

Matching aged gelcoat is a real problem. Twenty years of ultraviolet exposure shifts pigment unevenly across a unit, so a laboratory match to the original formula produces a repair that is technically correct and visibly a patch. We either tint to the aged surface and blend outward, or refinish the full panel in a modern basecoat system if the gelcoat has oxidized past the point where polishing brings it back.

Fairing is where these jobs are won or lost. A 40-foot sidewall in raking afternoon light shows every high spot and every soft edge in a filler transition. We block long, use long boards rather than short pads, and check with a guide coat repeatedly, because a repair that reads perfectly under shop lighting can look like a shadow on the wall once the unit is outside.

What is included

Tap mapping and moisture survey

The separated area is bounded acoustically and read with a meter before any cutting decision is made.

Resin injection with clamped cure

Low viscosity epoxy worked into dry voids, then pressed flat under cauls or vacuum until set.

Core and substrate replacement

Saturated foam and delaminated luan removed back to dry, sound material.

Stud cage repair behind the skin

Corroded or crushed aluminum members replaced while the wall is open.

Structural re-bond and seam glassing

Skin re-adhered under pressure, cut lines glassed, and the panel faired long.

Gelcoat repair or panel refinish

Tinted and blended to the aged surface, or refinished in a modern basecoat system.

Signs you need this work

• A soft or springy area on an exterior wall that moves when you press it with your palm
• A visible ripple, wave, or bubble in the sidewall that reads clearly in low angle sunlight
• A hollow drumming sound when you tap along the panel instead of a solid tick
• Fine cracks radiating diagonally from the corner of a window or a cap seam
• Chalky, dull fiberglass that leaves residue on your hand and no longer responds to polish
• Interior paneling that has lifted, stained, or gone spongy behind a cabinet