I spray my wings as I don't really like the look of clear wings. What I do is:
1. Trim the wing with a Dremel to the desired shape.
2. Get all the painting gear (I only use Tamiya
PS paints) ready to go (paint cans thoroughly shaken, cap off) and a clean cloth or paper towel.
2. Clean the wing thoroughly with hot water and a good amount of Dishing wash Liquid. I use a tiny bit of water to make the dishing wash liquid less sticky so I can easily rub it all over the wing (also on the overspray). Then I thoroughly rinse it with hot water and rub it while doing so. Your wing and your hands will be degreased thanks to this - it reduces the chances of the wing become greasy again.
3. Dry the wing with the towel. You can also let it dry on itself if you want to prevent one or two dust particles in the paint, but be sure to wash your hands again before painting.
4. Before painting, make sure the wing is well and truly dry, also in the corners.
5. Get the paint can (which was ready to go) and apply very thin layers: One quick pass over each surface of the wing is enough for one layer.
6. Let each layer dry in the sun or close to a heating source (radiator). That way you're ready for the next layer (as long as it's the same colour) within 20 minutes.
7. Repeat the layer process 3-5 times and you should have enough paint on the wing for it to not be transparent.
I used this process on many shells and wings, and I never had paint flaking off (not even
this wing that has thick layers of orange and white paint on the left... both below and above the black paint layer as I wanted the splatter effect to be visible on both bottom and top of the wing).
I did have paint rubbing off over time, but that's no surprise. The preparation seems very extensive, but it's really not that bad. All you have to do is walk to your painting gear and back every once in a while. Also, you can do 2-3 wings in one go and you'll spend your time more efficiently.
PS: Do not use white spirit, thinner or other solvents on the polycarbonate. Even if no traces become visible of a reaction, many of these liquids harm the strength of the material, polycarbonate is pretty sensitive to all this stuff.