Hi Martin,
I read a good tip for doing black outlines on wings and tails such as the early Pfalz E types that might work for this one. Use a wide tip Sharpie permanent black marker and hold the flat tip at a 45 degree angle to the wing, and drag it around the edge. if you are careful and keep it at the proper angle it will make the same width stripe along the entire wing and requires no masking. Only clear coat it with acrylic if you use lacquer the ink will bleed all over the light blue and ruin it! Would be worth practicing on a scrap wing or piece of sheet plastic to see if it works for you.
Dave
