RE: Surface coat
No it'll all have to be one or the other. If you use a polyester gelcoat, you have to use polyester resin for your CSM or cloth in your mold lay up. Not a problem to then use epoxy or polyester for your final part item. Or do one of each. But you can't mix materials in the same application. Making the the mold out of polyester and the part out of epoxy, I've found to be the most economical way of making things. Quicker too. The polyester "goes off" pretty quick so when you're building up a mold on a plug, you can just keep on putting layer after layer, no need to stop and wait hours for the resin to "go off".