A good affordable way to go is epoxy primer the chassis saver extremely durable and will not break the bank
After having done several car undersides with various products, I feel compelled to add my 2 cents. I have used POR 15 on many things, including car frames and it looked wonderful when the surface was prepared properly. You don't just apply it over a rusted surface, but must clean it with their cleaning product and then acid etch for proper adhesion of their paint. If done correctly, it looks close to powder coat.
I have used PPG DP 90 on rusted metal and it also works well on a cleaned , but rusted surfaces. On my current project, a 79 Malibu wagon, I cleaned the entire bottom of the car with cleaner and power wash, then brush painted the underside with PPG DP 90. It adhered well, but the brush made it difficult to get in all the nooks and crannies. This was all done in my driveway in a residential neighborhood over a plastic drop cloth. The fuel tank had been removed and painted POR silver. In the end, I wanted to get all the missed spots, so tried Eastwoods Rust Encapsulator aerosol for that job. It easily reached all the spots I had missed with the brush and provided a nicer finish than I had achieved with the PPG and brush.
If I were doing it over, for ease of application and final appearance, I'd just use the Rust Encapsulator. If I were building a show car. I'd remove the body, sandblast the frame after susp disassembly, clean and prep as required by the manufacturer and apply the POR. I prefer the look of the gloss black, but that is an individual preference. It comes in many colors, so contrasting color choices remain for the control arms and differential, for example. They call it powder coat in a can for a reason.
In summary, there are many products that will get the job done, you choose the level finish and amount of prep you are prepared to do for your particular job. If you are simply looking for fast coverage of a surface rusted area, including inside doors, that will look decent, Rust Encapsulator aerosol should be a consideration. If you want a good base to hold a top coat, PPG DP epoxy primer might be what you are looking for. (As a side note, I have found adding 1/3 more hardener to the PPG will give it a semi gloss finish. This is not recommended if there will be a top coat, but works fine as a final finish.) If you want a high finish on a rusted surface and willing to do the prep, POR a good choice. In the past, I have always brushed on the POR, as it flows very well and leaves no brush trails. There is my 2 cents worth.
