First, I use 100% Silicone. DAP brand is about $5 a tube, but WM ALL Purpose Silicone is $2.37.
You can cast a skim coat and lay a few beads of tube silicone (It will Adhere) after the skim cures, then fill over the cured Tube stuff OR you can lay out a bunch of beads, let them cure then lay them on the cured skim coat, as filler or dice them up and mix it in with some RTV.
Be advised, the tube stuff will float in the RTV.
In any event, the two are compatible, and using the tube stuff as a filler is a practical alternative to the very expensive RTV. The key is to make certain the skim coat totally encapsulates the model.
Instead of foam, you might want to try a Urethane Resin. You'll need micro Balloons to regulate it's buoyancy.
Here or Here.
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