Not that this matters in the application being done here but I was in between coats of paint the first Time I got on.
Here are the main differences between epoxy resin and Polyester resin.
Less activator required.
Thinner bodied so easier to us on some parts.
Poor adhesion to wood and steel.
Fair adhesion to itself.
High in solvency so easy to have air bubbles.
Heavy bodied less trapped air.
Requires higher strength activator (some 2:1, on high side)
Excellent adhesion to like product.
Excellent adhesion to wood.
Fair to good adhesion to metal.
30-60% more flexible.**
Less film (50%) to equal same strength.**
Higher heat tolerance.**
Now for the **:
There are different grade of epoxy fiberglass as it gets down to how much
Talc, chromium the manufacture ads to thicken up. If he uses cheaper heavy talc this will weaken the film and take away on flexibility and heat resistance.
If they blend with fine chromium or fine and medium grade talc of course it will be stronger.
Transoms are sandwiched in wood; this is why I would find it hard to believe
Any boat builder would use this, since polyester does not like to stick to wood.
I sell a boat builder and the smallest boat they make (all sea going) is 65Feet.
About 5 years ago an automotive paint warehouse special ordered 5- 55 gallon
Drums of polyester glass for a jobber selling to Martin/Lockheed they refused it because it was not epoxy and I thought I could help out and sell it for them to my boat builder.
He told me he had to sign an affidavit with the insurance company saying he would only use an epoxy.