Will
I dont think Epoxy is materially harder or softer than other resins. Its just differient chemistry.

Most stuff made professionally out of epoxy, like Hall Masts, Forte beams, etc, uses an epoxy resin that requires heat to cure it. The Forte stuff is cured in an oven at 300deg F, and Hall uses an oven too. Since thats not feasible for the homebuilder, West, MAS and others sell epoxy that room temperature cures. The downside is that room cured epoxy gets soft at between 120 and 140 deg F. A black part left in the sun, in TX on a hot day will get to 160 deg F, and according to Tony Delima at MAS, you want a TG of 40-50 deg above the working temp of a part, so room cure epoxy wont work for black parts that have to work in the sun. Now, if you cool the part down in the water (like a rudder) and then stress it you should be fine, but its something to keep in mind. A white part can get to 130 deg, so painting your part keeps the temp down quite a bit.

Also the first time an epoxy part is first heated at an elevated temp, it goes through a secondary cure, so you get a higher TG if you leave an epoxy part in the sun, or in your car. I usually leave my parts in the sun to cure, or put them in my car to get a higher heat cure.

If you want to make a black part, and its going to need to be stressed in a structural way (like a beam) in the sun and you want to make it with epoxy, you need to use some of these higher heat cure hardeners like, Infucure HT, Anhydride, Dici, or Low tox. West's Proset line has higher heat property's but isnt clear, so you cant make a black carbon part with that stuff.

I havent gone the route making an oven, Im toying with getting an old oven and putting it in the garage for small parts.

Steve Clark build an oven out of plywood and electric baseboard heaters some thermocouples. Its not that far outside of the basement builder's reach.

Sorry for draining the pond on that answer, but I was just talking to Tony about this yesterday, so its still fresh.

Bill