The problem is the paint you used, it's not formulated to adhere on polycarbonate (Lexan) plastic. What happens is that the paint can't flex along with the body so it cracks and since it doesn't adhere as well it flakes off.
The solution is to use paint made for use on Lexan. Several companies makes it but if you need spray cans then you should go to your LHS and buy Pactra or Tamiya paints that say for use on polycarbonate or Lexan bodies. If you have an airbrush there are more choices such as Parma's Fascolor, Pactra, Kyosho, Tamiya and Createx.
Another thing you can do is to litlle scuff the inside of the body with steel wool before washing it with dish detergent to remove the moulding release agents. That gives the paint a bit more tooth to adhere better.
Try it again but use the right type of paint. Stay away from hardware store paints since those are mostly enamels and are for exterior use or house walls and trim.
