I was made aware of this stuff by Noel on another forum. I have used it on one boat now and will definitely use it again. It seems pretty tough and it really made it easy to fill the weave on a rather loose weave cloth I was using.
I thinned mine with mineral spirits to the consistency of thick paint and then used a brush to apply it. It went on smoothly and in some places it penetrated all the way through the cloth.I applied two coats. It doesn't get brittle but leaves a smooth, flexible, but hard finish.
I also built it up on the stitching on the stems to seal and smooth the ends.
I painted mine, took less than a quart because the paint did not have to fill the weave.
If I remember correctly it took about 3 tubes of the stuff to do a 17 1/2 foot kayak. So that's about the weight it would add.