First time painter--Painting some atv plastics. Sanded with 120, adhesion promoter and 2 coats of ********** epoxy. I can still see the 120 scratches. Going with a metallic base coat.

How should I proceed? Try sanding with wet 600? If this doesn't fix it, will I need to sand it off and start again? Should I start with 400 and then go to 600?

I would guide coat it with something then carefully sand with some 400- 600 or even 800 just depends how bad it is. if it doesn't fix your problem you'll need to put something on there to sand smooth. more epoxy or a build primer, but keep using your guide coat.
Jay D.