We are done with the initial rough sanding on our red oak floor, and are moving up to the 60 and 80 grit passes. Time to fill. This floor is pretty tight, but there are of course some hairline gaps and nail holes to take care of. And in one fortunately small and inconspicuous area, the end fits are quite wide, on the order of 3/16 inch at the widest, perhaps from a sloppy old spot repair.

Our floor supplier set us up with Woodwise trowelable filler, in the correct red oak base color, enough to cover the whole floor. We also have been bagging and saving sawdust from the sanding runs since we got down to the bare wood.

Question is, does it make sense to mix the saved sawdust in with the filler? Especially for filling in the big gaps.

I did a test with just the filler on a scrap piece from where I had replaced a cupped board. The filler did fine on the scratches and nail holes. But I also tested it against a sawcut that was set up to be comparable to the gaps at the bad end fits. In that deep and wide gap, the filler slumped as it dried. I wonder if dust mixing might help prevent that slumping tendency. But most of the references I see to using dust refer to using it in a varnish base.

Not sure if it's important or not, but for the record, we plan to go dark on the stain, using Dura Seal's red mahogany. We did carefully test that against the same wood that is in the floor, and it came out looking fantastic. I am going to test against the dried Woodwise filler this afternoon.

On the Woodwise container, it mentions that larger gaps, holes, etc. will usually require filling twice or even three times. You can just spot fill those areas. Trowel fill the entire floor then when dry, spot fill any larger voids or depressions. You can do this in reverse as well. Spot fill large depressions first, wait to dry, then trowel fil. Either way. Really no need to mix the dust with the filler. I haven't tied it with the latex fillers. We used to mix edger dust with a laquer based product. Worked pretty good.l