Why Wet Layup Still Earns Its Place

Hand layup has been around since the 1940s and remains the method most small UK workshops reach for when a part needs making this week rather than next month. No autoclave, no press, no exotic tooling — just a mould, dry reinforcement, mixed resin and a little discipline. The catch is that wet layup is easy to do badly. A laminate that looks fine while it's wet can cure into something heavy, resin-rich and full of trapped air, and the difference between a good part and a bad one is almost always process, not product. Get three things right — resin-to-glass ratio, batch size and consolidation — and your parts will be consistent enough to sell.

Nail the Resin-to-Glass Ratio Before You Wet Out

Resin is there to bind fibres and transfer load; it is not structural filler. Every extra gram of resin adds weight, adds cost and adds brittleness, because cured resin cracks far more readily than glass. For hand layup, aim for a laminate that is roughly 40–50% glass by weight, which means about 2:1 resin to glass on a simple woven cloth layup and closer to 1.5:1 once you get confident with a squeegee.

The reliable way to hit that is to stop guessing. Weigh your dry glass first, then weigh out resin to match — say 300 g of cloth, so 450–600 g of mixed resin. Keep a digital scale on the bench and use it every time. A common failure is mixing resin first and then wetting out whatever cloth happens to be nearby; the laminate ends up at 3:1 or worse and nobody notices until the part is on the scales.

Mix Small Batches and Work to a Clock

Resin in a pot is a slow chemical reaction with nowhere to dump its heat. The bigger the batch, the faster it exotherms — a 1 kg pot can gel in ten minutes while a 250 g pot is still workable half an hour later. Mix what you can use in about fifteen minutes.

• Use shallow, wide containers rather than tall ones; more surface area means more heat escapes.
• Catalyse accurately. Polyester and vinyl ester typically take 1–2% MEKP by weight; epoxy hardeners go in at the ratio on the tin, often 100:30 by weight. Eyeballing catalyst is how you get a sticky part or a smoking pot.
• Mix for a full two minutes, scraping the sides and base of the pot. Unmixed resin shows up later as soft patches.
• Split a large job into stages — skin coat, then two or three reinforcement layers per batch — rather than trying to wet out the whole part in one go.

Roll Out the Air, Not the Resin

Trapped air is the enemy of strength. A laminate with visible white patches or bubbles between layers has a fraction of the interlaminar strength it should, and it will show up as a delamination the first time the part is loaded. Consolidation is a physical job and it takes longer than most newcomers expect.

• Lay the cloth, then stipple resin through it with a brush — don't drag, which pulls the weave out of line.
• Work a consolidation roller (a paddle or spiral roller) from the centre outwards, at right angles to the weave, then diagonally. Change direction each pass.
• Use a squeegee to chase surplus resin to the edges and remove it. If the cloth floats, you have too much resin.
• Roll every layer, not just the last one. Air caught under layer two cannot escape through layer five.
• Finish with a peel ply or bleed layer if you want a bondable surface and a slightly leaner, more consistent laminate.

Control the Workshop, Not Just the Laminate

Resin behaves differently at 12°C than at 25°C, and a cold workshop is the most common cause of slow cures and resin-rich laminates. Aim for a consistent 18–22°C, keep resin and hardener at that temperature for a day before use, and avoid working in draughts that carry dust onto a wet surface. Humidity matters too; epoxy in particular will blush with amine bloom if you lay it up in a damp, cold room. If your workshop is cold, warm the resin gently in a water bath — never flame or a hot plate.

Record What You Did So You Can Repeat It

Consistency comes from measurement, not memory. Keep a simple laminate record: cloth weights, resin weights, catalyst percentage, ambient temperature, gel time, number of layers and roller passes. After five parts you will have a set of numbers that reliably produce the same thickness and the same mass, and a heavy part can be traced back to its cause in minutes. Wet layup rewards patience and a notebook more than it rewards expensive materials — master the basics and a modest workshop can turn out parts that rival anything bagged.