I know there's usually a big mystery behind paint and body generally, so I'll be documenting that. There are many ways of doing it, of course, but projects seem to disappear from the forum only to reappear completely painted and finished.
Panels must be clean! Here are some products I use below.
Filling holes in plastic and fiberglass, I drill out the hole one size to remove any paint, clean, then mix epoxy with SM fairing compound. The compound makes it thick and easy to sand. 24 hours to cure, and tape or plastic over holes to eliminate sag. I've had the best luck with this method in the past.
As much as I would like to do perfect metal work, I can't. I use USC All Metal for roughing in gaps, and welds. It's a catalyzed aluminum filler. Light media blast, then applied and sanded.
After roughing in the body work, D/A sand with 80 grit, I cleaned, then everything gets a coat of epoxy primer. All panels must acclimate, along with the paint to follow the tech sheet. You can't bring a 50 degree panel into a 75 degree room and start painting. I use a temperature gun to make sure everything is up to temp.
Filler first or second is a big argument, but I want my metal sealed first.
Everything will get a complete skim coat of body filler next.