I am making a small mould and wondered how many layers of fibreglass to use on the back of the gelcoat? Also can I put all the fibreglass on at once or do I have to let it dry between layers? Thanks!
You do not want to put all the fiberglass on at once, there would be to much heat from the resin, you would have to wait about 2-3 hours between each two layers you put down.
I always find these kinds of questions tough to answer, because there are so many types of composites materials. “Layers of fiberglass” doesn’t really specify anything at all.
A layer of 4oz cloth is very different than a layer of 36oz. So what are you using?
Generally try to not lay up more than 30-40oz/sq yd on the mold at a time. Let it pass peak exotherm (hottest it will get during cure) before laying up more plies.
Total thickness of the mold will depend on thickness/complexity/quantity of the final part. A good rule is 3-5 times the thickness of parts being made.
Well here’s what I have been using for my small moulds…
Epoxy Gelcoat(wait 1-2hrs)
1 Layer Surface Tissue
1 Layer 200gm Plain Weave Glassfibre
3 Layers 300gm CSM
I layer 200gm Plain Weave Glassfibre
I’m using epoxy resin because these moulds are being used to produce CF parts and I read that polyester resin was not really suitable for this job. I also lay up all these layers at once because I understand epoxy does not have the exotherm issues like PE. I’m not sure if the above method is the “right” method but it does seem to be working for me. I have recently learnt that CSM is not compatible with epoxy, but like I have said I have not had any problems yet 
stiffer moulds are better. As a rule of thumb I generally build the mold with 3 times the layup of the product in thickness.
for the layup: begin with a light mat, then double every layer in wheight with curing in between(200 grams,450 grams, 2 times 450, etc.). Usually within 24 hours, that way you have some chemical bonding, rather than mechanical only, saves sanding. Use an unwaxed resin.
Hmmm, I take a gelcoat, then a skin coat (can be done on the same day) then slosh on 5-6mm of Rapid Tooling System (Nord Composites).
The 2.4 meter boat mould is made that way (Susho knows that mould) and larger moulds need more layers of 5-6mm. (A recently completed 54’ mould has 4 layers. (ca. 25mm thickness, 1")