Microballoons are tiny hollow glass spheres. I can't imagine they contribute much beyond making the mix thick. Wood flour is at least a medium strength thickening agent. I keep a can of West's 403 fibers for insurance. It makes lousy fillets but it's cheap and strong.

: I'm running out of wood flour. I'm unlikely to have enough to fillet and tape
: the inside shear seam. I've already done it in the center of the boat, but
: am waiting to cut the hatches to complete the seams in the two ends. Do I
: need wood flour for this, or can I use micro balloons? I was thinking of
: getting micro balloons for the end pours and wonder if it can do double
: duty. Or doesn't it have the bond strength needed for a shear seam fillet?