You should etch galvanised metal, there are several products your local body shop supplier can show it, if you don't want to etch it I would wipe it down with alcohol to degrease it, then wash it with vinegar (poor man's etching compound)and wet sand it with some 220 grit wetordry, then degrease and vinegar it again, epoxy prime it and paint it.


1999 GMC Yukon
1957 GMC 100 Panel