you can spray anything over almost any type of existing paint if you apply a seal coat first. A 2K epoxy is probably the best to seal the vehicle with then you can put whatever you want on top of it.

read up on spraying a sealer coat, usually w/epoxy its a slightly thinned version of full strength epoxy 10 - 15% acetone reduced, with that applied you can then walk away for quite awhile & not worry about surface rust occurring which I'd think in your area could be a real problem. you can do filler work on top of the sealer as well. regardless of what you do - read a LOT before doing anything, you'll be happier with your result if you do.

the clear is a polyurethane - see tech sheet - probably fairly decent stuff.

http://www.de-beer.com/TDS/en/TDS8-204_EC.pdf