"I do seem to have a problem with the paint not keying"
Yes, so do I. That's why I prefer the masking method - once it's masked you can prime and apply multiple coats, and if you're careful taking the masking off, it works a treat. The Eduard pre-cut canopy masks are great for starters (but a bit expensive), or you can make your own by putting oversized pieces of masking tape over the panes, burnishing it down to show the frame, then carefully trimming the mask with a very sharp knife. It's laborious but it does work - but only when you have a raised frame on the canopy. Alternatively you could mask off the edges of each frame with thin strips of tape and just paint the frame between them.
A method I used to use when I was younger was to paint a piece of sellotape or masking tape the right colour, then just cut strips and stick them over the frame lines. I've not seen it mentioned on this forum, and it doesn't produce such good results as masking, but in some situations, e.g. where masking is awkward, it might be the best thing to do.