When you want to prototype something, you just want the end result as clean as possible, so that's why i want to show you how you can paint polystyrene and have that really clean finish.
Step 1: Degreasing the Surface
You can use a degreaser that you buy in a convenient store, just make sure it's strong enough.
Step 2: First Layer : Plastic Primer
Here we use transparent plastic primer, spray from a 30cm distance and make clean strokes.
Step 3: Second Layer : Color Layer
Because of the use of plastic primer you can use regular paint, choose what color you want and again spray from a 30cm distance with clean strokes.