If you have a vented propane heater you shouldn't have moisture problems. I believe where moisture problems come up is when things cool there will be a moisture left behind as the metal cools. Getting everything toasty warm will help, as everything warms in the shop, tools, cars, walls, etc., the heat will stick around longer. This may take heating the shop for a few hours before you start working. I would get the shop warm, 72-75 degrees, shut the heater off, do my painting, wait until the paint is set up fairly well, then turn the heat back on for a few hours. I don't much like vent free heating, moisture, gasses, have to go somewhere. 99% efficiency is just that, but there are not a lot of btu's involved. A ceiling mounted heater would be my first choice, run copper for propane, a thermostat, vent through the roof, a fairly simple job to do. If you were in a colder state this might not work too well but you don't get terribly cold in Wa. Turbo has the right idea for his set-up.
"When I was younger, I could remember anything, whether it had happened or not." - Mark Twain