Do you have access to a compressor and spray gun? I have found PPG DP90LF epoxy sealer/primer to flow out and cover better than any other epoxy I've tried to date. Epoxy is the sealer of choice for rust containment or when painting over older painted surfaces. The PPG DP can be sprayed as a single coat sealer/top coat if a) it will not be exposed to direct sun b) you use the hardener. Its a very nice black satin /suede finish...perfect for frames. If it gets a lot of sun it will get "chalky". It is designed as an undercoat but many shops use it as a single product solution for frames. It works on old paint just fine. If you have rust remove as much as is reasonable, treat with a liquid ruts converter and seal.
There are plenty of other epoxy sealers but none lay down like DP and have that perfect sheen. And yes you can topcoat it. I just shot my Model A dragster frame with DP and then top coated with a suede black urethane paint....the frame will be exposed so I needed the paint to fight off the ultraviolet. The axle and rear suspension are only covered in DP90LF.
And that is my .02
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My Christmas present to ME! Bought Sat 11-30-2013. 72, 350 4 spd, Silver with blue interior...Can't you just hear Pablo Cruz playing in the background?