Mark,

One thing to keep in mind is that polyester resin shrinks as it cures while epoxy does not. Because of that, it might not be possible to remove the epoxy casting from a mold designed for polyester.

Another consideration is that epoxy cure is an exothermic reaction. Depending on the amount and shape you cast, you may need to have some kind of cooling mechanism available.

Epoxy doesn't need a gel coat since there are no pores to seal, as in polyester. If cast against a smooth surface that it can't bond to, epoxy will end up very smooth and glossy.

Laszlo 

Hi Thanks for your reply Laszlo,

Ah yes I remember polyester shrinks now. I have a post curing oven which i use to post cure epoxy, if i used this to heat a polster mold how much of the shrinking problem would this solve?
Do most people use epoxy moulds for epoxy mouldings this seams very expensive. I live in the UK and have access to a bio resin supplier, the resin is about 95% linseed oil and is UV curing, I've used it on some other projects with good results, the resin is a fairly new product but I have been assured there is minamal to no shrinkage with it, (it costs less than epoxy too).
Its interesting I don't need a gel coat with an epoxy moulding this seams a good weight saving too.
I plan to use only very light thin glass so I am hopeing the exotherm wont be too much of a problem.
My plan is to create a sandwhich constuction moulding, with a layer of balsa as the sandwich material, to do this i'm planning to lay glass on to the balsa and then vac bag it straight on to the mould, ( now I know no gel is required).
Is there any special mould prep for epoxy? i'm used to using wax and PVA with polester.

Thanks again, Mark 

Mark,

You are now beyond my area of expertise, especially concerning that bio-resin. Here's a link to the West System's booklet on vacuum bagging with epoxy resin. With any luck it'll answer your questions.

Good luck,

Laszlo