Yep, it was the heat. I was working inside the garage, but it was well over 100 degrees that day. The paint shop guys say it happens at those temperatures. Solvents in the epoxy can't escape fast enough before the surface of the paint hardens. The result is blistering.
I sanded the top coat off and waited for the temperature to drop down to 80. I used the same paint and the same equipment again, and this time it turned out okay.
