A good primer sealer like Dupont DP-90 will actually seal against weather, and is one of the best you can buy, but it'll cost a few hundred bucks a gallon. Worth every penny too.
I'd suggest a primer surfacer to start, which you can block sand to help find low spots. Then you can either paint over that, or if it's going to be awhile, you could prime it with the DP-90, which is an excellent product and will buy you some time before you have to paint.
For example, if you're straightening the body panel by panel, then DP-90 would be good to use. That gives you time to work on the rest of the truck until the whole thing is ready for paint.
