A lot of POR suggestions; but I know of an alternative, which I'm knowing is equally attractive; and for me, was the way I went : Eastwood, which can be found on Amazon or eBay, and has website too (Eastwood - Auto Body Repair Tools | MIG Welder | TIG Welder | Plasma Cutter | Auto Body Supplies & Accessories) sells POR products, i.e. POR-15, the POR prep products, etc. But they also sell product under their own brand name. This being their Chassis Black paint series. I too would have the chassis formally sandblasted (as universally agreed here, and I did as well). But then, one could go the route via Eastwood's 2K epoxy primer & then Eastwood's Extreme Ceramic Chassis Black on top of that. Or, use Eastwood's Rust Encapsulator product (which can indeed be applied directly onto the clean, sandblasted metal, as a rust-prevention primer) & then use Eastwood's regular Extreme Chassis Black on top of that (which is not only more smooth-finish than the Ceramic Black, but also incredibly durable in most every way). Both products are offered in gloss back & satin black - and will NOT quickly fade. For the internal portions of the chassis (inside the tubed cross member, etc), Eastwood has a Rust Encapsulator rattle can with a 24" long tube - that the spray will cover the entirety of all of these un-seeable places. I'd think the same type tube could be utilized for the chassis paint as well. - - You wouldn't go wrong with this route & product. However, attractive via Eastwood, they've got basically everything else you'd need as well. They'll even mail to you a product catalog each quarter, if you wish - that one can have a field-day looking through. ~Skydog.
This guy nailed it. Though you could go one step further and galvanize it as well. On our restorations we do the 3 step eastwood product. We also use a cavity gun and get the entire inside of the frame. Something powder can never do.