herc on the outside

On my xj their are a couple of surface rust spots on the bottom side of the rockers. I'm thinking about using my leftover herc. Can I just wire wheel then apply it as I did on the inside or do I need some sort of primer b4 the herc?

just make sure its clean and wire wheel - I did my rockers and lower doors up to the first body line 1.5 yrs ago and have had no problems with the herc not sticking.