IMHO (pro bodyman)..... I say strip everything to the substrate and start from new. That way you know your base is good. Especially since the PO painted it, you don't know what's hiding under there.
Here's my system for steel parts:
-Strip to metal. Can be done with 80g on sander, or chemically
-Repair dents. Filling small ones, or pulling large ones. Start with 80 and move up to 220.
-Spray epoxy primer (AKA non sanding, self-etching)
-Spray sandable filler primer
-Sand the primer after it's dry with 400g then 600g (800g if it's a light colored metallic)
If it's being painted black or dark blue, I would sand with 500g wet, then 1500g wet, then 2000g wet.
-Clean it. You can't get it too clean.
-Mask off everything you don't want painted. Don't forget the gas filler hole and underneath the tank. Dust comes from everywhere.
-Clean it again
-Spray basecoat on. Blending agent first if it's metallic paint. Wait appropriate flash times depending on what you use.
-Spray clear on.
If the finish it's too dry (looks like an orange peel) or has runs/dust specs; wet sand with 2000g and either re-clear or polish until you get it where you want it.
Plastic is basically the same, only use 220 to strip it, and wetsand it with 400g or 500g BEFORE you prime it. Not many people do this... and it makes a huge difference.
Prime the parts with a plastic primer, then sand with 400g up to 600g and then 800g.
To repair the plastic, 3M makes great products. It's like JBweld for plastic. You file out the crack, fill it, then sand it smooth and primer over top.
The rest is the same as above.
The end result is 100% based on what you do before paint. I always hear people saying "prep is 80% of the work"... but it's not. You could have the best painter in the world with the most expensive paint spray something that hasn't been cleaned or masked... and it will look like crap.
Take your time and it will be worth it in the end.
Good luck!
