Finally found someone to paint the interior of my dart. Didn't come out right and mostly my fault.

I've decided to learn and do it myself. Then I'll have only myself to blame. Omni was used, single stage. I need to smooth out some areas with filler and re-paint. Can I rough up the paint and use filler? Do I need bare metal? This is on the interior. If I can rough the paint for filler, what grade sandpaper?

Thanks for any help offered, Kevin