I use BobSmithIndustries 30min epoxy and denatured alcohol, usually thinned about 50/50. For me it takes a bit longer for the epoxy to fully cure, then if I just use straight epoxy. When it does cure and fully dry, it's rock hard. There's no "rubbery" about it. When I mix a batch I have more working time when I add alcohol which is nice for coating/painting it on smoothly. If I want a good finish, I use a good brush, If it's just a firewall I just use a throw-away acid-brush.
Climate plays a big part though with any type of resin like this. I'm in the middle of summer here in Cali, and temps are hitting over 100 degrees here in the valley, and everything cures faster no matter how you slice it. I do my work out in the shop before the afternoon heat hits me, but yeah.. some days it hovers around 100 in my shop. No problem with anything curing/drying here... and everything cures fast too.
If I'm using just straight 30min epoxy in the heat like that, I only have about maybe 17-18 minutes of good work time before it starts going off... Very doable, and I actually love using epoxies in this environment for that reason. I use 5min very rarely in this heat...work time with that is maybe 3 minutes. Fine for a quickie, but I'm never in any hurry anyway.