Do not use POR 15 or Chassis Saver on areas that are rust free or if you have treated the rust. Those products need corrosion and rust deposits to bond. Without rust, they are useless.
Eastwood's "Extreme" or "Ceramic" Chassis Black is an awesome product! Unlike Rustoleum and other general use products, Eastwoods Chassis series of paints are specifically for your vehicle. They are more resistant to heat and grease/oil. They have all sorts of different finishes and different ways to apply it.
Another great product is Zero Rust. It is a rust encapsulator/inhibitor, but unlike POR 15, it will bond to properly prepped rust free surfaces and prepped rusted surfaces. (by prepped I mean heavily sanded/fiberglass/bondo....) Zero Rust comes in spray cans and it's reasonably priced.
I'm "cleaning up" my 87 T. At the moment, the frame and suspension have been removed to get stripped/blasted, so I'm working on the underside of the bare body right now. Some areas are heavily rusted and other areas just need a little sanding and de-greasing. This is the perfect situation for a product like Zero Rust. I can prep the entire underbody and cover everything at once with one product. It's o.k. if I miss a few rusty spots when I'm sanding everything because it will bond just as well to the rust. Then, if I want to put a nice coat of body paint, gloss chassis paint or undercoating, I just scuff up the Zero Rust and paint right over it.
After my zero rust goes on, I'm going to scuff it and SPRAY it with Raptor. Raptor has a lot of characteristics of a bed liner/undercoating. The biggest advantage is that it has better UV qualities and it has a smoother finish than most bed liners. Also, Raptor will hold up MUCH better than most paints on street/strip cars.
The chassis will be media blasted. Immediately after getting it back, I will prime it and then paint it with Eastwood's "Extreme" Chassis paint. I've seen it on a few other frames and it is tough stuff and lays very nice!
I've talked to many people about different options and products for restoration painting and this is the way I'm going to do it. I'm sure there are similar products out there, but I named the ones that I'm familiar with.