I built up inner structures and then filled with syntactic foam. The mix I used was 3 gallons of low viscocity Epoxy resin and 7 gallons of 3-M micro-balloons. The bleded gooy mass is like warm Kraft Marshmellow creme. Cured, it is supposed to have a compressive strength of 3000 psi. The resin is no-blush 635 from US Composites. (they also sell the balloons and cloth) The "foam" can be easily sanded and shaped. The finished skin will be graphite & vacuum bagged. My intent is to have a truely water-proof structure.
Do you have a link or part number for your micro-balloons. I didn't see it on the site. Also where did you find the 3000 psi number