Paint not setting up.

Back in the 80s i had the engine compartment od a 68 mustand "professionally" painted by a shop.

They used a urathane. Years later if you set a tool on the painted area you would hear a snap when you picked the tool up..

The paint never fully cured.

Not much help to you. I would reread the instructions. Some times the mixture is by weight rather then by volume or visa versa