Have you been sanding the faces of the cast any? If not, you should.
The resin you're using (polyester) is a layering resin, meaning it cures in layers. What I do is pour a cast, let it cure for around a week, then sand through the outer (not fully cured) layer, exposing a cured layer underneath. When I'm speaking of layers, they are very thin, it doesn't take much. I start with and use 240 (wet sanding) until it is no longer tacky, then work my way up to 2000 (240, 400, 600, 800, 1000, 1200, 1500, 2000). After the 2000, it will be very hazy, but you go over it with some polishing compound (I use Meguiar's M105 and M205) and it becomes perfectly clear. You don't need anything powered to sand/polish these, in fact that might create too much heat, doing it by hand is sufficient. Use a rubber block sanding pad and put your paper on there.
As for the heat of the cast, you can control that by using less hardener. I buy syringes and needles (largest gauge I can find) at Tractor Supply (any place that sells livestock supplies should have them). They allow you to just draw the hardener from the bottle and get a more precise measurement. Once you know exactly how many cc's you need it's very simple to get the same amount every time. It will eventually eat away at the rubber stopper in the syringe, so use it for a few cast (if done on the same day), then trash it. The syringes are pretty cheap, 25 or 50 cents a piece, the needle can be reused quite a bit more.
