High detail fibreglass casting


Active Member
Hi all,

I'm trying to make a fibreglass casting from a silicone mould but I'm having trouble getting the matting to stick to the higher detailed areas. Also having trouble keeping the gelcoat even on the edges, corners and verticals. I've tried a few layers of gel coat, then thickened resin and then matting but still it's extremely hard trying to get the matting into the smaller detailed areas.

I've never cast something this detailed in fibreglass before. I was thinking of pouring in some polyurethane resin and covering the detailed areas, thus leaving me a flatter surface to add the matting to, but doesn't that defeat the object of using fibreglassing?

it's a large open back mould, so I don't think rotocasting would work either.
The glass will tend to lift up. You need to tap it down with a chip brush (cheap disposable brush). Just keep tapping it down until the resin starts to kick.
Practice, practice, practice...

The right working temps and humidity as well as tailoring the amount of catalyst to your work habits and the current environment is crucial...

You can use fillet material in tight areas to smooth them over, there is no defacto fillet material but most are a combination of the resin with some thickening agent, the thickening agent can be just about anything that doesn't react with the resin... Some popular thickening agents are cotton flock, milled fiberglass, wood dust or wood powder aka wood flour, talcum power aka Bondo, fumed silica aka Cabosil to name a few...

Also use smaller pieces of mat or fabric or use loose fiberglass flocking to better fit in tight areas...

And as robn1 said you need to work the material into the area, up down motion tapping from a chip brush works best IMO, but there are other options like rollers as well...

And last but never least, practice, practice, practice...
Do a search for bonding paste (that's what it is called in the UK). Generally it is used for joining fibreglass panels, but it is great for plodding into tight corners etc to soften them...making it easier to lay up the mat and tissue.
I am not nearly the expert here that other are, but when I make fiberglass casts from molds with detail I usually pour/brush on one to two coats of fiberglass, then spread out chopped fiber strands (Douglas and Sturgess: 1/4" Chopped Strands, 1 lb.) throughout the entire mold. I then apply several more coats of fiberglass to get a thick cast. In fact, the majority of fiberglass casts that I do, I use the chopped fiber. I feel it does a better job of getting in the nooks and crannies. I hope that helps.
Ok..try this.

FIRST and foremost, Go buy some GOOD resin. Most stuff at the Hardware store, is CRAP.

Here we GO.
One optional step is to Spray PRIMER onto your mold. Makes a great mold release, and your part comes out primered. Let it dry ALMOST all the way, you want it a TINY bit tacky.

Anyways...baby powder your mold lightly, blow out with compressed air. (Just like casting)

Mix 60% BONDO (Feather light) Paste with 40% Fiberglass resin.
Drop the appropriate bondo AND resin catalyst in and MIX thoroughly.
Paint the stuff into your mold over all your detail Use the CHIP brush to work out any air bubbles. DONT GO THICK, just a nice thin even coat.
Let it all kick a bit, but NOT all the way.
Now, a second "Mud coat" is next to fill in the recesses. You can use CABOSIL, ( a food thickener) or MARBLE Dust, or even BABY POWDER. (DONT LAUGH it works!)
Mix it to the consistency of PUDDING.
Kick it, and paint it over your detail trying to make a flatter surface to drop your matt into next.
as soon as your layer is done...take your ALREADY torn pieces of Matt and Lay then over your mud layer.
Mix some resin and catalyst and work it into the Matt.
Let it all dry and WALLA!

Note: MOST people OVER KICK the resin, it kicks too fast and you dont have enough working time. Keep this in Mind, FIBERGLASS eventually kicks WITHOUT catalyst.
So, as long you mix the RIGHT amount in, and mix it well, its going to kick.

Its better to go LIGHT then too heavy.

Hope this helps.

Thanks for all the info guys.

I'm waiting for some cabosil and going to give that a go. I tried it with some plaster powder and it seems to be thickening the resin up a bit.

Considered moving my mould around and letting the resin cure over the verticals to give them some strength.
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