I have used Saran Wrap to cover the fillet and by pressing on the fillet with your finger or a piece of flexible tubing you can shape it. Just leave the Saran Wrap in place and remove it after the resin has hardened. It is a problem getting the plastic to form to complex curves such as around the coaming without wrinkles but I have managed to get most of it smooth.

By adding milled glass fibers or wood dust to the epoxy you get a structural fillet (glass fibers are more structural than wood dust), but you cannot spread this type of fillet by the more conventional methods.

A fillet with micro-balloons is great where structural is not needed and is easy to shape by spreading with more conventional methods.

Doug