I'm looking for a material that is similar in consistency -when cured- to the shell of a walnut. In its pre-cured state it should be spreadable like clay. One possibility is a form of paper mache consisting of cellulose insulation, glue and joint compound. I really like the idea of using paper mache given its sustainability, but I readily admit that it's pretty off-beat as a material for housing construction.
So why do I need this material? As a coating to create a smooth rounded layer over the flat-surfaced triangular faces of the roofing grid system. Additionally the material should have enough heft to solidly grip nails or screws securing the covering shingles. Watch this video to get a clearer sense of the need.