Hey there man, I may be able to help you. Down here in Mexico, poliester resin is the usual thing since we don't have that many materials, so I might be able to give you pointers.
First: Polyester resin is super ugly. It's good for large casts, art pieces, and small stuff that won't touch food, skin, or the environment in general. It's used in fiber-glass castings and shells as well. Health-wise, it's very dangerous, as the fumes of the catalyzer are cancerogenic. so, take care. It's also brittle. The heat it produces is also very high, and if you mess with the catalyzer there will be shrinkage.
Now, is it all lost? nope. First: Are you using it straight? Polyester resin is almost never used just like that. first: You need to fill it. A filler, is basically ANY inorganic dust you can mix in. There are a lot of them, and some are specially made for very specific purposes (like micro-spheres, which increase thixotropy). Here are some:
Talc: Yup. the usual talc. Calcium carbonate. It's mid-weight, can be very well mixed (mix slowly, beware of lumps, use a sifter) it adds a lot of weight.
Santocel: It's a very light dust, santocel is the brand name in Mexico, not sure if you'll find it there. this one's REALLY dangerous for your health. it makes the resin very porous.
Alumilite: Very heavy fine dust. makes sanding easier. Good stuff!
Marble dust: Yup. Cheap, get it at building supplier's. AWESOME texture and great for simulating stony surfaces. Great for simulating marble (duh) since it's just crushed marble. VERY heavy, big fan. Reduces surface smoothness though.
Sand: Yup. sifted sand. Super rough, good for things that will be outside, add UV protection so it won't yellow if you'll ever use anything polyester on the outside.
Metal powder: Ever heard of coldcast? you can use heavy metal powders with translucent, crystal poliester resin to create a thick paste (a thick paste that's really filled to the brim with fillers is called a Gel coat usually) that will react just like metal when buffed. Add the awesome effect that you can stain and patina it with vinegar, just like metal!
There are also the fact that you can make cheap translucent casts with polyester resins. We call it "crystal resin" here in Mexico. You can even go crazy with effects, I enjoy for example creating Marble-like plinths for my miniature painting with marble dust, crystal resin in two batches, and some talc. I mix the talc and marble dust ( in heavy concentration) in the batches, (the talc stabilizes it and prevents the marble dust from just setting in the bottom) and mix some white pigment in one, black in the other. I catalyze both, pour them in the plinth mold, and stir a bit (not too much, so they'll have swirls and will mix JUST a bit). I then sand and polish. The crystal resin allows some light to come into the plinth and sparkes with the marble dust a bit, and it gives off a surprisingly good facsimile of real marble, for dirt cheap. Polyesters are good for many things.
Be mindful that even well done and prepared, polyester resin is no fun to be worked and cleaned for potential modelers. it is best for bases, heavy stuff, and scenery.
Now, ALL OF THAT was just about how to properly prepare it. Now, let-s really get on what you asked: Painting. You can pretty much paint polyester resin normally, and if it has fillers and has catalyzed completely, all should be ok. here is what you should to to ensure good painting:
1- Never paint a cast just out of the mold, Wait a day. and if possible: let the sun heat the cast, that'll accelerate any catalyzation action. Actually, models catalyze fully a week or two sometimes after they're completely hard, so patience is a good thing.
2- Once you're done, you want to de-grease and wash the parts. warm water and soap works. rinse, let dry 100%
3- After this, PRIME. This is super impotant and whatever paint you use, NEVER SKIP IT. Use automotive primer, cheaper, better.
4- Again, let dry. Do it on a sunny day, avoid moisture.
5- NOW you can get to painting with well thinned paints of your choice.
Also, you might want to try this technique: spray paint with primer your mold and then cast. That bonds the primer to the resin, and I've had mixed results with this.
Well that's my two cents. And I rambled. let me know if there's something else you need.