polyesther resins are used on boats, surfboards, automotive etc. epoxy resins are stronger and more flexible and will not attack styrofoam like the poly resins will. epoxies are also used in these same applications, but are generally more expensive, so the poly is used where cost is more important than weight and results. we use the micro-balloons or corofil in some applications doing repairs on full scale aircraft on the composite surfaces. i have also used them doing fiberglass repairs like mentioned above to thicken or lighten resin (epoxy or polyesther) when using it for a filler, but like mentioned it would seem like a rather heavy filler for most rc applications especially where something else would suffice.