First welcome to the board we have a few member from England.
I used Rustoleums rusty metal primer in the bed of my truck straight out of the can and the coverage was great, don't see a problem using flat but make sure its for metal and outdoors usage. I'd try a test piece to see if it needs much thinning.
BTW I used the foam rollers.
Good luck

Last edited by 69DartGT; 11/29/06 10:54 AM.