Epoxy

General Discussion
1

First of all, Don’t use polyester resins directly onto EPS foam! It will melt the foam.

That being said, there are applications where top coating a board that was glassed using epoxy resin with a polyester topcoat can be useful. I know a few glassers who like using color pigments to do art on aboard. They prefer to use polyester resin because they are familiar with the flow, blend, and workable time. If they have multiple boards they are finishing, they will use Polyester topcoats over both polyester resin and epoxy. I wasn’t sure if that was a good idea, but they didn’t seem to be having problems with it. I’ve been thinking about playing with pigments and had not seen any of these glassers doing pigments with epoxy. I would prefer not to use polyester resins…   The manufacturers of the pigments say they are for both polyester and epoxy resins, but I haven’t seen anything on how the ratios will affect cure times, and bond?

I found this research paper on bonding polyester to. For you who have to know the specifics…

2

Despite the title of your post, there really is no such thing as an “epoxy board”.  Usually the term refers to an eps core laminated with epoxy resin.  

You can use polyester resin over an epoxy lam or fill coat, but you can add tint and pigment to epoxy resin, so it isn’t necessary if its just color you’re after.  There is a lot of color stuff you can do with acrylic paint on foam before glassing also.

In my experience polyester resin over epoxy can bond, but there can be issues with peeling later on down the road. I myself prefer to finish with epoxy, if I have glassed with epoxy.  I don’t use polyester resin or spray finishes, but I’m not after that mirror gloss either.