Its light weight spackling from the hardware store. Its used for dry wall repair. There are two kinds. One is heavy and is obvious, the other is very light weight and will also be obvious. I just mix tap water to it until its a little thicker than milk then rub it into the weave with my fingers. When it drys it will be lumpy, just sand it lightly down to the weave. You dont want a layer on top of the glass, just fill the weave. Keep doing that until you're satisfied. Then prime and sand. And you're ready for finish paint. A really good paint to use is warbird colors. I havent used it yet but will be on a P-47, J3, and a L4 pretty soon. The big benifit of warbird colors is its high pigment content. Takes less paint. Perfect paint "the brand" used to be like that. Its not longer made. This makes a very light finish when using 1/2oz to 3/4oz glass.
Edwin