Microballoons add volume and buoyancy to your cast part. If you are looking to make your part lighter, have it float in water, or to use significantly less resin try using the Microballoons. General formula for adding Microballoons to Alumilite resins:
0-20% microballoons (by volume) will sink 25-30%,microballoons makes it neutrally bouyant - suspend 50%-100% microballoons makes it float
Disclaimer ... design and inner through wires, size and weight of hooks, eyes, and anything else will vary the weight and affect the buoyancy of the lure.