I have a vac canopy from squadron and the instructions tell me to paint a clear decal sheet, cut it into thin strips, then apply to the canopy frame. Is this because there is a problem painting straight onto the vac canopy? I'd prefer to use my usual method of masking the windows and spray painting in situ.
Advice would be most welcome.

My guess is that's mostly because it's easy to accidentally cut through not only the tape but also the thin canopy as well. I've never had a problem masking and painting vac canopies the regular way.
My advice is to use thin strips of tape to mask the edges of the clear panes, cut off the excess with a sharp knife, and then fill in the centers with larger pieces of tape. That way you can minimize the amount of cutting against the canopy, and also the risk of damaging it or accidentally cutting through. Just go easy with a fresh knife blade and use only enough pressure to cut through the tape.

Thanks Eetu. I will heed your advice and proceed with confidence!