The link states that the fenders come primed:
"Steel fenders are pre-drilled, primed and will require finishing"
& that's the way I've always seen replacement fenders....is there a problem with the factory primer(many parts from overseas have had bad primer in the past) ?
If the primer from the factory is good,then finish coat should be all that's needed or maybe some surface primer to correct imperfections & then finish coat.