You would probably want to use the finest grit necessary to smooth out the finish, probably some 220 grit would suffice and still give enough "tooth" for the next top coat. No need to take it all the way down to 80 grit, you would just need to sand out those scratches after.
I honestly wouldn't try and strip it right down unless you had too many layers of paint or an adhesion problem, you could end up doing more harm than good.
I agree with ShortFuse, another thin layer of primer to block the different paints, and then you new top coats