That sucks. I had a magnetic blackout panel on my '10. nothing like that happened to my car. But I took it off every time I washed the car and made sure the area where the panel sits was thoroughly dry before I put it back on. Am I correct in assuming that this is the 1st time you took the panel off since you first put it on?
Don't sand and prime. If it did this to surface paint, it'll just make it rust with primer underneath. Scuff it a little, hit it with a couple coats of clear in of a can or rust paint to seal it and stick the panel back on. You can properly refinish it at another time.
Are those marks deep scratches, or paint that has been completely removed? Or is it just resin form glue?
Hard to tell from the photo as to the extent of the damage.
My decklid panel was on for over a year, adhered with double stick tape. I worked it pretty good and gingerly with goo be gone and a plastic putty knife and got it off.
I washed the car, clayed the car, then I went over the entire decklid area with Pinnacle Paint Cleansing lotion, a Porta Cable7424xp and white Lake Country White Polishing CCS pad (has some cut to it).
After that I switched out to a Lake Country Orange CCS Pad (moderate cut) and Pinnacle Swirl and Scratch Remover (a compound). After, switched to a new White CCS pad and followed by Pinnacle Advanced finishing polish and then a nice caranuba wax.
Removed all the scratches, swirls, putty knife scratches into the clear-coat... etc.
If my scratches were really bad, then I might do a test spot with/wet-sand with 3000 grit or higher 3m automotive sand paper
Depending on how bad, if that is just residue from the panel, you should be able to get it off. If those grey lines are primer, you're done.
If it's paint / clear-coat failure, you have no other choice than getting it to a body shop and have it repainted. I wouldn't put the decklid panel back on as you'll get rust... If that is bare metal exposed.