Underpressure. the propblem with clear coating bare metal is that it wont ad hear very well to the surface and will flake off. you are better off being meticulous about oiling the metal to keep it from rusting. there are a number of old school hot rod guys who rock bare metal cars. its a lot of work but can be done. Prime example is Brian Bass from Texas. He did a 3000 mile road trip in his bare metal Model A last year and it doesn't have any oxidation on it.