I just pressure washed the frame on my s10 and used some Rust Tough Enamel in semi gloss and it turned out great. I wouldn't worry about sanding the paint on the frame but the fenders I probably would. Oh and that POR-15 is great but $$$. The stuff I bought only took 2 cans to do the whole frame and it looks really good IMO.
2005 Silverado SS AWD
AEM CAI, ARH LTs, HD-2 shift kit with vette servo, Circle D 278mm 3200 stall Madtunesracing.com street tune (MTR now has a Dynojet 248 chassis dyno)
13.82 @ 94.92 w/1.83 60'