If you have the car to bare metal, I would use a two part etching primer. If you still have old paint, well afixed, then a single stage sealant would be good for all. It is far less expensive and is a very good base for more work. As far as old stuff in the paint, wash it, wash it, wash it. Cleaniness is next Godliness. Do you plan to out live the paint job? There is a lot of stuff out there that may do things promised but end the end it all come down to elbo grease. Oh how I wish it wasn't so.