I usually don't use the liquid masking agents.
I've tried them, and have found that using Bare Metal Foil as a mask is much easier, more efficient, and I get better results.
I've found that I get slightly uneven edges with the liquid masks, no matter how carefully I apply the stuff. The BMF is easy as can be - apply it, burnish it down, trim out the frames, burnish it down again, then paint. Takes 5 minutes to do a complicated canopy like Catalina or B-25 or B-29.
Generally, if the canopy is closed in the model, I attach the canopy to the plane, then mask and paint.
Try it - you'll never use a liquid mask again.
Cheers
LeeTree