I had the same problem with my wifes 68 F250. I used U-Pol catalyzed primer as shown at the link below:
http://autobodystore.net/Merchant2/m...egory_Code=PRI
It can be rolled on, so there's no overspray to deal with and goes on very thick so there's a lot of film thickness to block out. It's been sitting outside unprotected for about 3 years now and looks like the day I put it on.