As A rule of thumb always use what works.. and I will add when asking people in the know look at there master peices and then make a realistic choice..
Micro balloons : two choices Cabicille or Ammicille Both are designed as a Balloon or and epoxy holder
One adds strength other is more flexiable and can sand easier..
Use the balloons. If you would like to purchase there are people who sell Microballoons here at Rcu..
Not that expensive AnchorBond brand Is 5.00 for a Quart
Ratio as much ballons you can add till it gets to be like a Tooth paste substance..
any other questions email me..