Ineer Fender Prep/ Painting?

['70 C/10 SS500]

This past weekend I pulled the front apart to finally replace my battery-acid eaten pass. side inner fender well. I have a new Goodmark fender well that I assume might be EDP black primered? If so, how should I go about painting it? Should I just mildly abrade the primered finish to give the new primer a surface to hold onto, then seal and paint, or should I just paint right onto the factory finish? Any tips for spray can woes? I'll be painting semi-gloss black then wet sand and polish. I hate painting, never seem to do as good of a job as I envision in my mind, any tips to produce a quality finish with spray paint from a can? For this fenderwell, what grit sand paper to use for primer and finish coats? Am I turning a mole hill into a mountain? Thanks in advance!