Microballons are micro glass/phenolic ballons that look like a powder, and can be added to epoxy, etc..., to create syntactice foam. All that means it that you end up with a product like the epoxy used here, that also has lots of little (too small to see) bubbles in it, and is therefore lighter than epoxy, easier to sand, bulked up, etc... Often you have to add small amounts of thickener to the epoxy also, but that seems to be unecesarry with 5 mins, since they are already pretty thick. Any boatshop supply should sell you some balloons, and you will need so little they might just scoop you a sample.