Well, here is what works for me. This is a narrative I put together for my nephew who had an interest and wanted to know how I did it.
Sand out (300-500 grit) any imperfections, sprue tabs and boogers.
Use spot putty or similar to even out low surfaces, then sand again as needed.
Wash in warm water with a little dish soap, rinse well and leave to dry (don’t wipe) for 24 hours. (Use a dollar Store toothbrush to get in the nooks and crannies when washing.)
Put it on a stand and prime with actual primer, I like Rustoleum 2X Gray Primer
Sand again with 800-1000 grit max.
Wash in warm water with a little dish soap, rinse well and leave to dry (don’t wipe) for 24 hours. (Use a dollar Store toothbrush to get in the nooks and crannies when washing.)
Repeat steps 1-6 as needed until the surface is smooth and consistent.
Paint desired color using light coats, less is more!!
NOTE: How much paint to spray can only be learned by practicing it. Find other plastic items to hone your skills on. Styrene food containers and covers were great items for me until I gathered "extra" model bodies to practice on.
YOU COULD STOP RIGHT HERE AND CALL IT GOOD IF YOU LIKE.
OR-
Sand again with 1000-2000 wet/dry paper. Sand right at the sink with warm water running on it. Fold the paper as needed and get in all the nooks and crannies.
Rinse and let it dry (don’t wipe) for 24 hours. Toothbrush again when rinsing.
Patiently paint another coat the desired color using light coats. Coats being a paint and sand session.
STOP RIGHT HERE AND YOU SHOULD HAVE A GOOD SMOOTH SURFACE.
OR-
Sand again with 2000-3000 wet/dry paper. Sanding at the sink with warm water running on it.
Fold the paper as needed and get in all the nooks and crannies.
Rinse and let it dry (don’t wipe) for 24 hours. Toothbrush again when rinsing.
Now you are ready to shoot it with clear (adds shine, seals decals) OR add another color coat (then repeat 12 & 13) OR polish with polishing clothes (4000-12,000 grit).
If you chose to shoot clear, you then could repeat steps 12 & 13.
I have never done more than three color coats.
Apply wax to protect it and keep the shine in it.
Dust proof display cases is worth EVERY dollar to protect your model and keep it looking great.