How to Laminate a Repair Properly
Most bad laminates aren’t caused by bad materials. They’re caused by skipping the boring parts.
This one’s a simple polyester repair — closing up an old port light opening on Paikea, foam-cored, nothing exotic — but the basics behind it are the same basics most people get wrong. Not because the technique is hard, but because it’s tempting to rush straight to laying up glass and skip the surface work that actually determines whether the repair holds.
Working with what’s actually there
Real boats aren’t flat, clean, or uniform, especially once they’ve had thirty years and a few previous owners’ worth of repairs done to them. This job had three different substrates to deal with in one small area: the new foam core, filled and sanded flush; the original factory laminate, laid up in a heavy 800g double bias with all the texture that comes with production boatbuilding — high where the stitching sits, low where it doesn’t; and the factory tape join, stepped up abruptly with no taper, leaving a real gap between the tape and the new foam.
That gap is the part that matters. Laminate straight over a gap like that and you create a void — a spot where the new glass never actually touches or bonds to what’s underneath it. It looks fine from above. It isn’t fine underneath. Voids like that are exactly the weak spots that show up as failures years later, and they’re avoidable with a bit of care at the surface prep stage.
Sanding for bond, not for beauty
Surface prep here was simple — 60 to 80 grit on the orbital, enough to knock down the highs without chasing every low spot in the weave. On a polyester boat like this, the existing laminate surface, even slightly aged and a bit grubby, still bonds well. Chasing every low point with a finer grit to get a “perfectly clean” surface costs you more in fibre and structure than you gain from the marginal improvement in secondary bond. The trade-off isn’t worth it. What actually solves the texture problem is a different trick entirely.
The trick that changes everything: a filled resin bedding layer
Mix your resin with a thickening agent — cabosil is the go-to — until it’s a workable paste, and apply it as a thin layer directly onto the surface you’re laminating to before the glass goes on. Applied wet-on-wet, the filled resin squeezes into every low spot, every gap, every uneven transition, and gives you a smooth, consistent surface to laminate straight onto. It has to stay thin — this isn’t a gap-filling exercise, it’s a surface-levelling one — but done right, it removes the need for hours of extra sanding and eliminates the voids that would otherwise form over every dip and step.
This isn’t a polyester-only trick. It works exactly the same way with epoxy. Any time you’re laminating onto an imperfect surface, a thin filled bedding layer solves problems that sanding alone can’t.
Weighing resin properly
For hand lamination without a vacuum bag, the simplest rule holds: your resin weight should at least match the weight of the glass you’re wetting out, plus a bit extra to cover what stays behind in the brush and in the initial surface bond. Weigh the glass, weigh the resin to match, and you’re not guessing.
Learning to listen, not just look
Glass gives you a visual cue when it’s properly wet out — it goes transparent, and you can see straight through it to whatever’s underneath. Carbon doesn’t give you that. The way around it is to train your ear on glass first. A laminate that’s fully consolidated and free of trapped air makes no sound at all when you squeegee over it. A section with air still trapped underneath makes a distinct crackling, snapping sound as the squeegee forces the air out. Learn to hear that difference on glass, where you can also see it, and the same audible cue carries straight over to carbon, where you can’t see it but can still hear exactly the same thing happening.
Squeegee angle matters too — working at 45 degrees to the weave gives a noticeably better result than going straight along the fibre direction, which tends to roll and distort the fibres rather than press them flat.
What it looks like when it goes wrong
Paikea’s interior has its own cautionary example built in — old cover-cloth laminate with visible bridging, air pockets where the glass wet out but never actually made contact with the surface below. Press on it and the fibre gives, because there’s nothing underneath holding it. It’s not adding meaningful strength to the structure. It’s cosmetic glass with none of the job it’s supposed to be doing. It’s held up for thirty years mostly because the surrounding structure was overbuilt to begin with — not because the repair itself was sound.
That’s the real argument for taking the extra ten minutes on surface prep and the filled bedding layer. The difference between a laminate that’s structural and one that’s just there to look finished isn’t the materials. It’s whether it actually bonded.