What kind of resin you should select depends on the job you are doing. Polyester resin is excellent for fiberglass lay-up--building a boat, for example-but it is less ideal for repair work. Polyester laminating resin doesn't fully cure while exposed to air, so when it is used for lay-up, every application joins to the previous one on a molecular level. You end up with what is essentially a single substance encapsulating multiple layers of glass fabric.
When you are doing a repair, you need for the resin to also function as an adhesive-gluing the patch to the surrounding surface. Polyester is not very good at this. Epoxy resin, on the other hand, is a first-rate adhesive. Because of this, as a general rule you should use epoxy resin for all repair work.
The exception is when the repair will be finished with gelcoat. Epoxy adheres tenaciously to cured polyester but the reverse is not true. me bond between an epoxy repair and an overcoat of polyester gelcoat will not be strong. Use polyester for repairs that will be gelcoated. Cost can also be a consideration. Epoxy is 3-5 times more expensive than polyester. This is relatively insignificant for a small repair, but where the damaged area is large, using polyester resin significantly lowers the cost of the repair
Grinding (sanding) is essential regardless of which resin you choose, but if, for reasons of cost, you elect to make your repair with polyester resin, it is imperative to grind the repair area especially heavily. First wipe the surface thoroughly with a dewaxing solvent, then use a 36-grit disk to grind everywhere you want the resin to adhere. Heavy grinding can somewhat offset polyesters weaker adhesion.
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