Since epoxy is mixed at a specific resin/hardener ratio which is the same for all applications, concentrated amounts of it will heat up, but usually not to the point of damaging anything - although you probably don't want to hold the cup through the hardening process. What can make a big difference though is the addition of certain fillers, like microballoons, which due to their hollow, air-filled nature tend to insulate, trapping heat. The cup containing a concentrated mass of these fillers can heat up to the point where it is smoking.
Polyester resin is a different story. It will contain a promoter (often some sort of cobalt compound, which gives the resin a slightly blue tint, though there are a variety of other promoters). The MEKP liquid hardener oxidizes the promoter in the resin, generating heat and the heat hardens the resin. A thin layer, like you would apply when sticking down a layer of fiberglass might do well with maybe ten to twelve drops of hardener per ounce of resin - giving adequate working time and a fairly quick cure. Gelcoat might need nearly double that, as it will tend to peel as it hardens if it cures too slowly. A thick casting of the same resin could be done with much less hardener added (maybe four drops per ounce, although ambient temperature also figures into all of these). The reason is that the heat is concentrated in the thick casting's limited surface area. The thin layers, like the fiberglass, take more hardener because the resin is spread out over much more area and dissipating the heat the hardener and promoter are generating.
If you have a fair amount of leftover polyester resin in the cup after applying a thin layer like fiberglass or a filler coat, and it was mixed with a lot of hardener, it will really heat up big time. It may smoke, it may crack the hockey puck as it hardens and it can even catch fire.
The yellow color on my Guide is Ace Hardware polyurethane floor enamel, rolled and tipped. I just looked through the color cards until I found one that was kind of a soft shade and had them mix it for me. It has a bit more orange peel texture than some of the expensive marine enamels, but not bad and not very expensive. Another good and reasonably priced custom color offering is Home Depot's "One-Part Concrete and Garage Floor Enamel". I used it for the base color coats on my fur trade stripper canoe. The base was rolled and tipped and the shading was sprayed, the grain lines were applied with a grooved roller I made, and then the flat finish top coat was Enduro Pre-Cat conversion varnish which I sprayed. You can buy a custom mixed gallon of these sorts of enamels for about what a quart of the premium marine enamels cost.
