Good grief, you gotta pull the fenders to get those flares off? I thought these trucks were made by chevy, -not ford! Ok, rant over......
I'll second Robyn on the rust mort, -it is great stuff, -and really works. I resurrected a '73 chevy cab (and we all know there was lots of rust to deal with), and used that stuff in areas where it wasn't practical to cut and weld patch panels, it worked excellent. As extra proof that it works, I know where that truck is today, -and there isn't any sign of rust-related paint problems, -and it was done over ten years ago.
Another product that I used to seal up the treated/repaired spots was PPG "DP" epoxy primer. It ain't the cheapest stuff, but it adheres and seals very good. And because it is a catalyzed product, it is very durable. Not that it matters, but when using the PPG epoxy primer, no sealer is required for top-coating or base-coating, -it's that good.
The newer version of the stuff is called DP/LF. It isn't quite as good as the old stuff, but is still some of the best stuff you can buy.
My .02
1994 K2500, 6.5, NV-4500, 4.10's, T-Master, GM4 at 18, Cranked DB2 at 5 BTDC, 599 block .020" over, decked .010", ARP studs, 18:1 Mahle marine pistons, SCAT 9000 steel crank, HV oil pump. Dual-stat/130GPM, C2 boost/pyro, 3"x4" Exh, ATA I/C in process.
NO MORE DS4, NO MORE PMD'S! I RELOCATED MY PMD TO THE TRASHCAN!