What are the Differences Between Polyester Resin and Fiberglass Resin?
Strictly speaking the original fiberglass resin was polyester resin. The problem is that the industry evolved but the names for the complete material didn’t. Formally fiberglass is a composite material composed of glass fiber (similar to the glass fiber in insulation) and a binding agent the resin.
The first resin used was polyester resin and that is what you find in vintage cars (classic corvettes), vintage surfboards and even some vintage instruments. As the material evolved and started being used for more types of products and needed different properties they changed the type of monomer in the resin. Other things where added to give the composite better flexibility (frequently vinyl ester), stronger (epoxy resin) or cheaper (styrene monomer).
Now what they call fiber glass resin is a mix of monomers for the cheap stuff you get at home depot 50/50 polyester/styrene. The good stuff is pure polyester or epoxy. If you are doing your own custom mix you can get the polyester, styrene, and vinylester as pure components and add the amount you want.
Epoxy is usually done pure or mixed with other additives as the epoxy is more expensive and you want to maintain its superior properties by using more expensive additives that match its chemistry better. I know several surf board makers that do this where they mix their own custom resin blend that works best for their products.