Stripping to bare metal, although costly, is the best way to ensure that the paint system you choose will adhere, thus eliminating any compatibility issues. There are a number of ways to achieve this goal, including any of the many types of blasting (media, bead, baking soda or walnut shells), chemical stripping or sanding. Depending on your budget and the end result you are seeking, all of these scenarios are good choices. Additionally, stripping to bare metal exposes any sheetmetal repairs, ie body filler, replacement skins etc.
Good luck with your nice 3 window cab.
