I don't know if it's included in the links posted by Chris, but, in this scale, especially on the shape of the Heinkel canopy, it might be better to use tape for the frame work. You can use Tamiya tape, or even ordinary 'Sellotape' (Scotch tape, I think it's known as in the U.S.A.).
Put a strip on some glass, or similar smooth, clean surface, paint the tape in the desired colour and, when totally dry, cut it into suitable sized strips to represent the framework. Apply to the canopy, pressing down firmly but gently. When done, give the strips a coat of clear varnish, to seal the edges, then, when this is dry, go over the frames themselves with a semi-matt varnish to match the rest of the paintwork.
Again, it's fiddly, as everything on this canopy, in this scale, will be. But, it's probably easier than trying to mask and paint the frames, and provides a realistic result.
