: Does using microballons in the endpour and/or for fillet work make the end
: result stronger, weaker or just lighter?

: How much is too much?

I used microballons in my endpours and was also worried about strength. Since I made the endpours before the deck was on (S&G) I added plywood reinforcing in a # shape and filled the cells with a thick cream consistency. Turned out very light and although untested, strong. I'm hoping that if the epoxy mix fractures, the ply reinforcement would contain the damage to only one cell and not fracture the whole endpour.