A couple of things need to be said here.

If you're going to thin epoxy, use acetone. It's for thinning epoxy. If you must use alcohol use denatured. It's 100% alcohol and won't contain water unless you leave it open overnight.

Ultracote does stick to painted surfaces. Sometimes it helps to sand the painted surface. If the painted surface has been exposed to fuel residue, very little will stick to it. Unless you clean it very thoroughly. Acetone works good for this.

Polyester resin is what fiberglass used to be laid up with exclusively. It is a resin that is catalyzed with drops of chemical instead of the almost equal parts being mixed. It creates a very strong fiberglass that is appreciably more brittle than fiberglass laid up in epoxy. Boats used to be polyester resin fiberglass. They'd actually wear out as the stuff shattered internally. Boats are now mostly epoxy and still wear out but not nearly as fast.

Polyester resin is much cheaper than epoxy to build things with. It creates things that are way more apt to eggshell when crashed. It's less flexible and more brittle. A buddy of mine went through a bunch of fiberglass airplane models last year. Every one of them was not worth trying to repair after ANY kind of damage. They eggshelled all over. One big bomber basically did an flop landing and darned if almost every fiberglass part eggshelled. We walked up to it expecting to see the gear messed up and maybe a spar or such busted and found something that looked like it'd been hit by a deathray or sonic killer pressure wave. No doubt it was all polyester resin glassed.