2 K urethane high build

ir3333 said
Local guy i know used to do show quality work and i remember asking him why there was about 3/32" of primer in a clear jar on his bench? He told me when he primes a vehicle he puts that small amount in a jar and when it dries and cracks the car is ready to sand.
That was about 25 years ago!

That was probably the old lacquer primer surfacer which dried by evaporation rather than curing like a 2K (the 2K means two part as in a hardener). In the early 80's I had a small body shop on our family farm and that is all that we had back then. I would crank out a car every few days and they looked great but then I'd see them from time to time weeks later (sometimes they came back to me for this, other times I just saw them on the street) and the surfacer would have shrunk so much that scratches would show through the paint. I would contact the owner an have to redo the work. About the same time there was a National Geographic documentary on TV where they covered the process of an old Dusenburg or similar car from auction to restoration and the shop that did it waited 30 days between coats of surfacer and the job took 12 months due to waiting on the lacquer to dry like you said. 2K primer is a total game changer.