A lightweight, low-density epoxy filler. Not ideal for bonding, but good to mix with wood flour for lightweight fillets or used alone as a Bondo-like fairing filler on lumpy hulls that will be painted.  Perfect for ultra-light endpours in kayaks, or for building up solid epoxy structures without adding weight.  "Micro Balloons" are exactly what they sound like---microscopic phenolic spheres, such that most of what you're mixing is air.  Makes it super-easy to sand.  Every boatbuilding "larder" should include microballoons.