I'm planning on making a fin shortly in fibreglass and was wondering about resin choice. Based on availability I'm limited to polyester or epoxy.
As this is to be done as cheaply as possible I have 2 concerns:
1. If I go with epoxy, will I be able to make an effective mold from fibreglass and plain epoxy?
2. If I go with polyester, will the fin be strong enough?
I have made a couple of molds and stuck a few videos up here on how to do a mold, there on vimeo under Keith , I think
have a look at the vid's and forget about every thing I say then have a look at Tim hammer the white lion, he has some excellent vids on doing a mold 
Indeed you're right, I thought mould was the American spelling.
So if using epoxy, what's the equivalent of a gelcoat? Or does epoxy not need it?
Well I think it depends, polyester has the advantage of being harder and wearing better. But it's also more brittle, so if there is any flex in the fin, you'll get stress cracks through it. If you're using carbon, forget polyester, it gives way before the carbon reaches it's potential.
Epoxy is a safer bet, as it's tougher.
Select to expand quoteikw777 said..
"Mould" is the word you are looking for.
Use epoxy without a doubt.
thanks for the spelling lesson bright spark, you can now take a seat at the back of the class and we will call you when we need some spelling corrections 
I have used polyester /vinyl ester /and epoxy, for a hard wearing surface i'm considering on going back to vinyl ester, a couple of times I have left the lid off the vinyl ester and some resin dust entered the can and sent the whole can off
some epoxies are good but some are really ratio dependent
Thanks for the replies, so it sounds like I can use the same epoxy for the mould that I use for the fin.
Another question I have: I'm keen to get a nice surface on the fin straight from the mould, is PVA as a release agent strictly necessary or is it belts and braces for epoxy? That is, if I use enough wax, will that be sufficient?
there are some mold
release's that don't need pva and personally I wouldn't use them, if you try to use pva it will fisheye , I find the best way to use pva is spread it while wearing a rubber glove , just put down a thin layer and keep spreading it until it's even with no fisheye
Vinyl Ester for the moulds with mould tooling gel coat.
Epoxy for the fins.
Read up on finishing moulds to a perfect finish and proper mould waxing and just polish them well with wax.
FGI used to have some excellent instructional technical handouts. Not sure if they still do but someone in the industry will.
Good luck! 
Select to expand quotesailquik said..
Vinyl Ester for the moulds with mould tooling gel coat.pineapple said..
I'm planning on making a fin shortly in fibreglass and was wondering about resin choice. Based on availability I'm limited to polyester or epoxy.
As this is to be done as cheaply as possible I have 2 concerns:
daffy pineapple is looking for a cheap way out , tooling gel is exy , last quote I got was $55 per kg at 4kgs
if your making your mould from vinyl ester or polyester you need to do it in small layup's , probably about 6 per side to eliminate the heat and shrinkage
Haven't made a fin mold yet - however have done many for my other obsession
Quick simple tool coat in mold can be made using your laminating epoxy mixed with west cotton flock/colloidal silica and some tint (some use graphite)
In this forum Rcgroups composites lots of discussion on this plus experiences with release systems
This post has a useful video
www.rcgroups.com/forums/showpost.php?p=14323652&postcount=7
Cheers Jeff
Hi Pineapple,
PVA sprays fine with no orange peel if you run a fine spray and high pressure.
I use 90 psi with the feed valve on the gun closed well off to give only a very fine spray.
I do a couple of coats an hour apart.
Fin comes out of the mold with a nice finish.
Select to expand quotekeef said..sailquik said..
Vinyl Ester for the moulds with mould tooling gel coat.pineapple said..
I'm planning on making a fin shortly in fibreglass and was wondering about resin choice. Based on availability I'm limited to polyester or epoxy.
As this is to be done as cheaply as possible I have 2 concerns:
daffy pineapple is looking for a cheap way out , tooling gel is exy , last quote I got was $55 per kg at 4kgs
if your making your mould from vinyl ester or polyester you need to do it in small layup's , probably about 6 per side to eliminate the heat and shrinkage
You don't need much. See if you can get 250mm from someone who uses it in bulk. Do it cheap or do it right! 
How much do you pay for epoxy resin anyhow?
If doing multiple layups after allowing for setting, use unwaxed resins. Many GP polyester resins are waxed. Surfboard laminating is not waxed but filler and finishing resins are. Surfboard laminating resin is usually promoted "hot" so do thiner layers if you use it. Vinylester resin used to come un-promoted and unwaxed, but I think you can get it pre-promoted these days. Great stuff!
Select to expand quotedaffy has a good point
If doing multiple layups after allowing for setting, use unwaxed resins. Many GP polyester resins are waxed. Surfboard laminating is not waxed but filler and finishing resins are. Surfboard laminating resin is usually promoted "hot" so do thiner layers if you use it. Vinylester resin used to come un-promoted and unwaxed, but I think you can get it pre-promoted these days. Great stuff!
Te Hau said..
Hi Pineapple,
PVA sprays fine with no orange peel if you run a fine spray and high pressure.
I use 90 psi with the feed valve on the gun closed well off to give only a very fine spray.
I do a couple of coats an hour apart.
Fin comes out of the mold with a nice finish.
I have never used a high pressure spray , but have found a couple of good mold release's work great without pva but with pva they fisheye
I've also found moisture promotes fisheye, in saying that if you apply pva too wax that hasn't totally dried it will fisheye, bead, or orange peel no matter how fine you apply the pva
daffy has a good point, use unwaxed resins , the reason why is because the wax will work it's way to the top so you have to use a solvent in-between laminates
my last few molds have been epoxy / uni glass/woven roveings around the sharp edges and epoxy and sand blasting grit, sand blasting grit is excellent because it has been granularly graded washed and salt free and its cheep
