I've finished an extensive sanding process in order to stain and poly my floor and am only now seeing considerable gapping between the boards. It's just that in the last few passes much of the gap fill cracked out and came up with the vacuum. The gaps themselves aren't particularly big, just a good number of them.

I know you're supposed to do the gap filling toward the middle of the sanding process but I'm wondering if I can just do it prior to the final screening prior to stain. Is there any real risk associated with NOT filling them?

Gaps in the floor can collect larger than desired amounts of stain and either cause an extended dry time, activate of react with previous stains or finishes applied to the floor (if it is a resand).

I use a product made by woodwise. Their prefinished floor filler can be applied before staining, in between coats of finish, or after the final coat. You can find them on the web and order filler from the direct.

are the gaps on the longsides of the boards or the but ends? i ask because i have found that filling gaps on the longsides with latex filler can lead to problems when the floor begins to shrink and expand due to seasons changing. it tends to crack when the boards shrink then come out when the boards expand. but thats up in new england where are summers are very humid and winters very dry. never the less i have filled chips and but end joints after the floor is finished by filling with latex filler wiping off the excess leting the filler dry completely then applying 1 or 2 coats of finish with a dry bush in a feahtering motion. as for as longside spaces i would just live with them.