for your scratch: only hammer it low if there are high spots. sand with 80 or 180, then fill with evercoat, or another good body filler. stay away from bondo. use a hard block with 180 to get it perfect. now to prep for primer, sand all around the 180 with 320 or a red scotchbrite. the idea is to fill the 180 scratch, but you cant get any primer on shiney paint, just leads to problems in the next step. use an aresol etch primer under the epoxy primer, on all bare metal spots. this will replace the factory corrosion protection.
What I just described above should be done in one shot, beginning to end. once its primed, you can stop. only use an epoxy primer, a 2-part mix and a cheap spray gun. no rattle can primer, it'll just absorb moisture and ruin what youve done, and it wont build for *****. the idea is to build to give you something high, so you can block it flat with 400.
As for the skirt, I believe its urethane. to repair gouges and scratches, tear into it with 220, then fill it in with filler. scuff way beyond the repair to prime as well. if you end up with a lot of exposed raw plastic, youll need an adhesion promoter under the primer, but you wont need any etch prime(no metal to rust)
hope this helps a bit, if you need more advise just ask.
94 V8 on 17" custom paint, R.I.P
97 Mark VIII LSC yeah
08 Ford Edge, white for now