I hope nobody minds if I reply to this older thread but I just wanted to add my 2 cents, for what it's worth. I've used liquid masking agents before but, as others had mentioned above, none of them ever worked to my satisfaction. Instead, I kinda make my own now.
I use Elmer's white glue mixed with just a drop or two of water at the most. Then I add just the smallest bit of liquid dish soap to break the surface tension. After mixing it up, I use paint brushes and/or toothpicks to mask off those nasty canopies with all the framework. B-36 and Avenger canopies are no longer a problem for me.