Choosing the Right Carbon Fibre Method

Wet Laminate vs. Prepreg: Choosing the Right Carbon Fibre Method

Choosing between wet laminate and prepreg is one of the first decisions you’ll face on any carbon fibre project, and it’s one Shayne’s made a few hundred times over — from grand prix campaigns down to a single part on Paikea. Here’s how he breaks it down.

What’s actually different

The core distinction is when the resin goes on. With a wet laminate, you lay the dry fibre and apply resin as you go, on the job. Prepreg — pre-impregnated fibre — arrives from the manufacturer already saturated with resin, ready to use.

What Shayne’s doing on Paikea is technically a prepreg too, in the loosest sense — he’s making his own “wet preg,” pre-wetting the fibre out ahead of the job rather than applying it live. But it’s not what the industry calls prepreg, because it isn’t a frozen, manufacturer-impregnated roll. The real dividing line isn’t wet-versus-dry so much as it’s the resin system, and specifically, how that resin is made to cure.

A wet laminate — or wet preg — will generally cure at close to room temperature, down around 10-12°C, though Shayne likes to post-cure beyond that to get the properties he actually needs. A manufacturer’s prepreg, from someone like Mitsubishi Rayon, Newport Composites, ACG, or Gurit, needs heat — usually 80-90°C or more — to cure at all.

Why you’d choose prepreg

The big advantage is working time. A prepreg gives you weeks before the resin starts to go off, which means you can lay up an entire hull shell — fibre and resin all in one — without racing a clock.

The other advantage is accuracy. Prepreg behaves almost like a vinyl sticker: you can cut it to shape with real precision, which matters enormously for patching and overlaps. In Formula 1, they’re cutting to tolerances under a millimetre, with laser placement systems and CNC cutters borrowed from the textile industry. Shayne’s done plenty of this by hand too, back in his Seatec and Gurit days — cutting a pattern, numbering each layer, marking reference points so everything lines up exactly where it needs to when you’re laying it up.

Why it’s not always the right call

Prepreg comes with real downsides. It needs high-temperature curing, so your moulds have to hold up under heat and often pressure too — ideally an autoclave, for proper consolidation, though on the boat Shayne’s typically working at ambient or one atmosphere of vacuum.

The bigger issue, for anything with complex shape, is what Shayne calls “sleaze.” In composites, sleaze is a good thing and a bad thing at once — it’s how well a material will drape, twist, and pull around a complicated shape. Dry fibre has plenty of it. Prepreg, especially collimated tape, has almost none. There are tricks to force it — heat guns to warm the resin and get it to soften and flow — but it gets messy fast on anything with real geometry.

Then there’s storage. Prepreg needs to be kept frozen at -18°C, and even then you’re only good for about 12 months if you’re lucky. Short-term at 0°C buys you a month or two. Left out in open air, you’ve got about a week before it’s no good. That alone rules it out for a lot of jobs where a freezer and that kind of logistics just aren’t practical.

Which one to reach for

For one-off parts, prototypes, and anything with complicated geometry, wet laminate is usually the more practical and affordable choice — no freezers, no autoclave, and the fibre will actually go where you need it to go. For production runs, or anything where ultimate precision and long working time matter more than cost, prepreg earns its overhead.

Part of our composites series

For more on the fundamentals of working with carbon fibre and composites, see Working with Composites.