I will try to give as much information as I can about doing prep work to plastic and metal bodies for paint. There seems to be quite a few questions about plastic and prep work so here goes, and remember these are just my personal preferences so you may use any product or method you wish.
Plastic bodies:
The very first thing is you can either remove the body parts to be worked or you can leave them on and mask everything, it,s personal preference but removing the parts usually works better. After that you need to scuff the whole body with either burgundy scotchbrite or sandpaper about 240 -400 grit dry paper, make sure you just remove the shine with the scuffing and not sand like crazy.
After scuffing now is the time to wipe it down with some wax and grease remover, wipe it on with one rag and wipe it off with another. Now you can do any bodywork that needs to be done, for filling scratches and gouges a polyester based finishing putty works the best for that, unlike the heavier "Bondo type" filler the finishing putty sand easier and doesn't tend to leave pinholes.I usually sand the putty with 240-320 grit dry paper then wax and grease again, remember wipe on wipe off.
Now is one of the most important parts "Adhesion promoter", this is a product that is sprayed on that softens up the plastic to accept primers and paints, if you do not do this step you risk all material you put on from here on out flaking or peeling off, especially if you are applying graphics later. Even if you are using Krylon Fusion I would still apply adhesion promoter. There are many different brands but I use either PPG or Dupont, make sure you read directions for applications and dry times.
Now you can pick a paint system and prime, if you are going to use automotive paint than I probably would not use bomb can primer, pick a primer that you're finished product material will accept. Prime accordingly, usually 2 to 3 good coats will do, then I usually sand with 240 grit if you are going to apply more primer later or 400-600 grit if this is you're finish sanding, and if you break through the primer in spots I would re prime and then spray a sealer on before applying color.If you have wet and dry sand paper you can wet sand if you like but I usually just use dry paper.
Just remember plenty of scuffing, adhesion promoter and plenty of wax and grease remover steps and you're job should turn out just fine.
Kevin
