Just straight sand to a heavy cut polish. Every filament can be polished. It is the same amount of work as any proper high build primer but the results are usually better quality. I was an auto body painter for a decade and owned my own small shop. I know exactly what is possible with various primers and epoxies. You have far more control over details and edges with polishing. For pro paint, take it to 800 grit, spray a primer sealer and shoot it like anything else. There is no shortcut though. You must sand the surface smooth to perfection. Get decent sanding tools like a DA, pen sander, print sanding blocks, and get a ton of sandpaper. In auto body work, sandpaper, tape, and masking paper/plastic are some of the biggest expenses. Pro paint is 99.9% sanding and prep work. The only sandpaper that exists is 3m Imperial wet/dry. Everything else is a joke that clogs in 1/10th the same amount of time and is therefore 10× more expensive than it first appears.