From Tower Hobbies....

#1. item # TOPR1090....Supreme micro balloon filer.
#2. item # GPMR6165....Milled fiberglass.

Either one will work....mix with fiberglass resin, then put in the catalyst. Mix
again, and apply. The micro ballons are lighter. There is another product
called "cabosil"....it is very light, and makes a good puddy too....available
where ever fiberglass products are sold.

FBD.