Marty, I don't think micro-ballons will stiffen the epoxy. I use system three and I need to add the cabosil to stiffen the mixture. The "manual " that comes with the system three recommends using cabosil. I tried it without, using a tremendous amount of micro-balloons and it wouldn't stiffen up. It is my understanding that the mico-balloons makes the epoxy lighter and sandable and the cabosil makes it stiffer. Tom