Other than using "white-wash" years and years ago to give shading for a greenhouse, I have never heard of anyone painting greenhouse panels. Of course the white-wash was applied to glass, since polycarbonate was probably not even invented back then.
That's an interesting concept though. You would need a lot of cans of spray paint to cover all the square feet of polycarbonate paneling. A better option (if it works at all) would be to use "clear-coat" made for automobiles. At least you would know that the product has gone through many years of research and is actually made to withstand outdoor conditions.
