TOPIC: epoxy/polyester compatibility

This has been hinted at but I wanted to bring it in to the forefront. The new Glastron transom is coated with epoxy and I want to start fiberglassing it in. Can I use a Vinylester (or other polyester-type) resign on the epoxy without any compatibility issues?

polyester resin bonds poorly in a secondary (mechanical) bonding situation which consequently makes epoxy resin the resin of choice for repairs

gelcoat does bond to a properly cured and prepared epoxy surface. There are a couple issues to be aware of to have success making this repair. There are three situations that cause gelcoat to not cure over epoxy all related to the hardener chemistry. Epoxy hardeners are basically a blend of amines and amines can terminate the chain reaction formation of the radical molecule that is the basis of polyester (and vinylester)
cure chemistry. So by carefully mixing, curing, and preparing of the epoxy so that there are no unreacted amines to interfere with the gelcoat cure, gelcoat bonds quite well to epoxy.

The first situation is undercured epoxy. Gelcoat applied to undercured epoxy will be in contact with unreacted amines and the cure will be halted.
The second situation is if the epoxy is mixed off ratio so that it is hardener rich, again leaving unreacted amines free to interfere. Third is the issue of amine blush, commonly called blush. Blush is a surface phenomena that is a reaction of the amine molecules at the surface with the carbon dioxide in the air. It forms easiest in the presence of moisture, so working in cool, humid environments will maximize the formation of blush. Any amine hardener has the potential to blush, but it can be minimized by careful choices of amines in the formulation. In fact, WEST SYSTEM 207 Special coating hardener is one of the lowest blushing hardeners on the market and still maintains structural properties on par with our other hardeners. Regardless of chemistry blush is very easily dealt with because it is water soluble. A simple wash with clear water removes the blush. No soap, no solvents. Then sand that washed surface with 80 grit paper to provide the gelcoat with sufficient key so it won't run. Be sure to use non-air inhibited gelcoat that has a paraffin wax added. Gelcoat is applied over epoxy on a routine basis everyday in boatyards that are aware of these issues.

MP, In this situation, I'm not going to be gelcoating over the epoxy. This present application is on the interior and the finish is painted glass/resin. I am going to be putting probably 3 layers of cloth (in increasing sizes) at the corners and then I want to put a layer of mat across the whole transom. I've used epoxy with cloth, but never with mat.

There is no compatibility issue- As MP mentioned, it's a matter of the bond not being as strong between an 'ester resin and epoxy. Polyester resins form a chemical bond with the previous layer, kind of 'melting' into it a bit. They don't do that when applied over epoxy- and form only a mechanical bond. If you rough up the epoxy enough, it'll form a pretty decent bond- plenty strong for the job.

Epoxy has a much stronger mechanical bond than poly does- is there any reason you are not using it for all the repair work?

Os for using the mat- Make sure that you are using a mat that is intended to be used with epoxy- some aren't. Many mats are held together with a binder that dissolves in styrene (present in the 'ester resins). It can cause an incomplete wetout when you're using it with epoxy. I used biaxial on my transom to get the maximum buildup in as few layers as possible- I used a 'stitched' cloth rather than a bonded one, so I didn't have issues with the wetout.

-Andrew