Well if the fender is shipped in bare metal then usually it has some sort of oily coating that needs to be removed first of all. I usually clean it real good with carb cleaner, then wax and grease remover, but thats just me. Then I would reccomend sanding it with 240-320 grit, hit your edges with a red scuff pad, then give it some etch primer followed by your filling primer. Hope this helps!!