Will Gelcoat Adhere To Epoxy?

Epoxy is considered to be the choice for many marine repairs. Yet, the question often arises of ‘how well will gelcoat adhere to it?’ To have success there a couple of issues to be aware of, basically there are three issues of why gelcoat will not cure on epoxy, all related to the hardener chemistry.

Epoxy hardeners are basically a blend of amines, which can end the chain reaction of the radical molecule that is the base of polyester and vinylester cure chemistry. Therefore mixing carefully is a must, as is preparing the epoxy so that there are no un-reacted amines to interfere with the cure. Over-all gelcoat bonds to epoxy quite well.

The first problem situation is under-cured epoxy. If gelcoat is applies to epoxy before it is fully cured, the contact with the un-reacted amines halts the curing process.

The second situation would be that the epoxy was not mixed at the correct ratio, so that it is hardened rich, and leaving un-reacted amines free to interfere.

The third issue is commonly called ‘blush’ or amine blush. This is a surface phenomena that is a reaction of the amine molecules at the surface; a reaction with the carbon dioxide in the air. It forms easily in the presence of moisture, especially humid environments; so avoid working in cool damp locations. Any amine hardener has the potential to blush, regardless of the chemistry, but is very easy to deal with because it is water soluble. Simply wash with clean water to remove the blush using no soap or solvents. Then sand the area with 80- grit to provide the gelcoat with a textured surface for best adhesion. Be sure to use a gelcoat that is waxed.

Gelcoat does bond well to a properly cured and prepared epoxy surface.

(Note: information taken from an excerpt by Technical Services West Systems Inc.)