Clean the body.
First, you have to wipe clean the inside of the Lexan body with a lint-free cloth. This is to remove any particles of dust and debris. After wiping it down, use liquid dish soap and warm water with a non-abrasive cloth and thoroughly wash the inside of the body.
This helps to remove other contaminants like oily residue (mold releasing agent) that the manufacturers use to keep the Lexan from sticking to the molds. This releasing agent can prevent the paint from bonding with the surface. Nothing is more depressing than having this awesome paint scheme that you spent countless hours designing start chipping off.
Dry the body.
After washing the RC body take a non-abrasive, lint-free cloth and once again wipe down the inside of the body to make sure that all the moisture is removed. I always let the body air dry for about 15 minutes to ensure that I won't have any problems with moisture on the surface. If you don't clean the surface properly the lint, debris, and moisture will not let the paint bond right and those type of mistakes can ruin a professional looking paint job.
Scuff the painting surface.
Use a fine steel wool or very fine sandpaper and lightly scuff the inside of the RC body. This step can help the paint adhere better. Caution: steel wool will leave behind a lot of fuzz and debris so be sure to wipe the RC body clean after using steel wool. Use a very fine abrasiv because you don't want to gouge or deeply etch the body with scratches that will show through the paint. Don't do this to areas such as windows that won't be painted or other areas that will remain clear or translucent -- the scratches will show.
Mask the body.
After the body has been thoroughly washed and wiped down and left to dry for a bit you can start the masking procedure. If you are not comfortable or are a beginner not wanting to do a complex or multi-color design, you can just mask the windows off. After you complete the masking, paint the body a color of your choice.
If you are experienced or want a more elaborate, multi-color paint scheme you'll need to apply masking to all the areas not to be painted with the first color you'll use.
This is pretty much the process that you will go through in prepping a Lexan body for painting. A reminder: RC Lexan bodies are painted on the inside not the outside. The reason that RC bodies are painted on the inside is to keep your paint job looking good and also to keep it protected.
Have fun and be creative.