I used to use resin for my molds(good detail retention) but if you don't get the mixture right the molds can warp and crack. Resin costs a lot more than POP.
I've also had a couple of resin molds that didn't set completely.(what a mess).
I thin the epoxy down with acetone to a watery mixture. I put 2 or 3 coats of this mixture on and it soaks into the plaster really well. I put the molds into the oven at 150 for an hour. Then I make another mixture of epoxy thinned down with about 1/2 the amount of acetone I used first time, and apply a couple of more coats. Then back into the oven. When it comes out if it's not smooth enough, I put another thin coat on.
This works for me; it may not work for others. It will rob the finer detail but what I do is "over-do" the detail so that it shows when I'm finished.
Hope this helps.
www.novalures.com