So, I thought I would just throw this out here to see if anyone has any comments. When I first started building canoes, and looking into silica based fillers, my first thought was that one could use diatomaceous earth instead of the silica you can get from pottery shops. Freshwater diatomaceous earth (deposits of the skeletal remains of single celled critters) is used as a paint additive for flattening, and as a paint filler/thickener, among other things. The particles have a variety of shapes that supposedly improves the durability of paint. It is primarily composed silica, but has a bulk density roughly half of that of silica flour, which might be a nice property for those of us building 18 foot 80+ pound canoes.