Hi Parrot,
First, masking without seepage is difficult. I suggest masking and then spraying a barrier of Future. Many modelers dip the entire canopy into Future and let it dry, so Future is great for sealing the mask.
Next, I suggest only painting canopies with acrylics. My Polly Scale and Tamiya can be removed from the clear area with a sharpened piece of balsa wood or bass wood. These are soft enough not to hurt the canopy unless you are just brutal. Just chisel the errant paint with the wood.
If you do paint with enamels, and the Future didn't create a seep-proof seal, first try to remove the Future--it may carry the enamel with it. Future dissolves instantly with ammonia, and I have not had ammonia hurt any of my clear pieces. Should the enamel hold fast, then try to remove it with thinner and a fine brush, or thinner and the wooden chisel method.
If that does not work, take a piece of the clear sprue, sharpen it, and chisel with it. I read once that clear sprue upon clear pieces will not mar the piece as easily as regular styrene. I've done this with some success.
