I found this at the West System site http://www.epoxyworks.com/22/polyester.html There is a wealth of information (granted...it's probably a bit biased) at http://www.epoxyworks.com/ See the last few articles listed on the left for some great explanations.

Quote
Understanding the materials
Polyester resin laminates achieve their highest strength when the bonds between layers of fiberglass are chemical or primary bonds as opposed to mechanical or secondary bonds. The manufacturing process for polyester boats has been developed so that a chemical bond exists between the gelcoat and the laminate. When a polyester boat is built, polyester gelcoat is first sprayed onto the mold surface. The first layer of the laminate is then applied to gelcoat, which has not completely cured. The two layers eventually cure together with a chemical bond between them.

Applying gelcoat to a cured laminate relies on a mechanical bond. Because of the difference in curing chemistry, it is not possible to achieve a chemical bond between epoxy and polyester gelcoat. We developed some tests, to determine whether or not the mechanical bonds achieved between gelcoat and properly prepared, cured epoxy were strong enough to achieve a durable repair...


the test information shown on the site indicates only a slight reduction in bond strength between epoxy and and a polyester gel coat and they were really focused on boats that were left in the water (the rumour was that because the gel coat will absorb water and the epoxy will not, that the bond will suffer).

Last edited by Jake; 09/20/04 10:27 PM.

Jake Kohl