I don't know about affordable but I don't think you want to cheap out on the foundation you plan to put all of the work you are about to do on top of. I use PPG epoxy on bare metal and if it's going to sit longer than 1 week I put Featherfill high build primer on it. If you do it that way you don't have to worry about adhesion problems with the epoxy and you can come back to it a couple months later if needed. Now if the car is getting wet like you say just put epoxy on it but when you come back to it you will need to scuff the epoxy and put another coat on it before you can start working. You might want to ask this question here http://www.autobody101.com/forums/viewforum.php?f=5 this is what these guys do for a living