You can get phenolic microballoons.  The Gougeon Brothers sell it as light weight filler and has a rust-red color to it.  It's not as strong as glass micro balloons but is a lighter weight material.  However, 3M also sells different grades of glass microballoons.  We generally use an all purpose industrial grade however there are many more stringent requirements for the stuff and you can have stronger material with a tighter spec, lighter, smaller spere size, whatever. Much of it is even used in cosmetics.  The folks at 3M are very helpful but they only sell through distributors.

Kind of on a related subject, if you want SUPER STRONG putty, you can also get DuPont Kevlar pulp to mix with resin.  It is a very interesting material.  A very small amount makes the mix thixotropic but woe be the builder that needs to sand this stuff!  It a material sent by the Devil himself to those that build up an area to excess and need to grind it down!  Trying to spread it smoothly is like trying to spread chunks of toilet paper mixed with maple syrup.  Very lumpy but the stuff does have amazing strength and is light weight, (the specific gravity of Kevlar is 1.41)

Dan Newland