[U1timate PigDog]

Firstly I'd chek that it says suitable for polycarbonate on it, I belive there are some tamiya paints that are for a different purpose and dont work.

Given that its right, if your spraying a single colour just get a brasso or something that will lightly scratch up the inside of the body (Dont do the window areas)

Give another good wash with hot water and washing up liquid and give a good rinse. (sounds like youve done this step fine already but...)

then when spraying do light coats, light coat the body then dry with hair dryer/heat gun then apply next coat, follow that method until you have the required coverage so you cant see through the body when its sat on the floor.

Hope that helps.

Also Tamiya paint is the best in my opinion (nothing better out of a can)