The thing to remember when adding microballoons to either epoxy or polyester resins is this, the more balloons you add, the softer the mix will be and hence easier to sand. I find that the polyester resin and microballoons mix is easier to sand to a feather edge than the epoxy mix. Also, when the polyester starts to kickoff, the mix gets to a consistency like a cheddar cheese and is easily trimmed with an xacto knife or even coarse sand paper where it actually starts to roll off, just be careful at that stage. Also, when applying, make sure you pat it in to make a solid contact.

I often wondered how well balloons and bondo would be together, just a lighter bondo I always thought. I mean bondo is basically just polyester resin and talc. Glad to know someone actually tried it and it works.