How to bond: glass to glass, glass to wood...
Microballoons are hollow glass spheres. The material looks like a white powder. I have seen brown. The material is used as a sanding filler. It is mixed with epoxy or polyester resin to create fillets. Addition of the material to the resin makes a past that can be used to fill holes, create fillets and use generally where a filler is used. The more microballoons in the resin the easier it will be to sand. However if the mix is too dry it may not adhere too well. Microballoons do not add strength to the resin. It is a lot lighter than automobile body filler.
Hobby stores carry it in small amounts. I believe SIG markets it. I purchase mine from a Fibreglass products store. It cost me $15 CDN for a four litre container.
Ed S