I have never used primer unless on bare aluminum. When I re painted my old jon boat, there were some bare spots, but I didn't bother to prime them. Just scuffed everything with sand paper and a scruffy pad with paint thinner. Cleaned off everything with alcohol and let dry. Used Rustoleum exterior enamel, both roller and rattle can. That was over 6 years back, still looks as good as the day it was painted. No loose, no flaking, no chipping, but some scarf marks from dragging it over sand and rocks. It does sit under an awning out of direct sunlight which I believe helps a lot!!