Microballoons are just that. Incredibly tiny little bubbles of a lightweight material that are added to epoxy or other glues as filler.

The way I usually use them is for creating a Fillet between say, the horizontal stab and the fuselage.

You mix epoxy, add microballoons to get a fairly thick consistency, spread it into, onto the joint and shape it.

Once cured it sands much more easily than straight epoxy, and you haven't added much weight to the tail of your plane.

Dennis-