Wow, your right...this is an assignment. I still say that the Rep was wrong...sorry, I'll try and explain why. If the paint was so hot and the paint flashed over before the thinner could evaporate it wouldn't blister, it would solvent pop...and the fact that it almost went away when the sun went down tells me it's not thinner, it's expanded air...one way to get air to expand is by oxidization or off gassing and it doesn't take much oxidization to create a gas between metal and paint. Even if you trapped thinner, how would putting DP40 over top seal it and stop the bubbles? Because it was so hot, could it be tiny bits of water vapor from the compressor?
This may need to go on "Unsolved Mystery's"...LOL...but I can't see it being trapped thinner...that would cause a pop, not a blister...I could see it being moisture from an over heated compressor...not droplets...atomized moisture going through the paint gun.
One more stupid question... did you go, bare metal, body work, DP40, paint or bare metal, DP40, body work, primer, DP40, paint...Or how did you guys do it back then?