Hi Paulnchamp
It is actually pretty simple. I make a puddle of white glue on a margerine tub lid and stir in about two drops of dish washing detergent, just enough to break the surface tension of the glue. Then I use a small brush and paint the glue carefully on to the clear panels starting in the corners and working my way along the edge. after the edges are done, I use a larger brush and paint in the area inside the edges. Clean your brushes up with water. When it is dry, I use the larger brush and do a quick second coat. After it drys you can airbrush away knowing your clear parts are sealed. To remove the masks, I flick a corner up with a toothpick that I sharpen up like a chisel, then pull the mask off with tweezers.
Works for me and way cheaper than those liquid masks which are probably the same thing as you are making with the glue.
Cheers
Bob S.