In article , "doozer"
says...
While people are already thinking about filling gaps in floors (see an
earlier post about cork beading) I have a similar question.

We have, courtesy of the previous owners, sanded and varnished
floorboards in our living room and hall. The gaps between boards range
from basically nothing to about 4 or 5 mm. My SO doesn't mind them but
they bug me because I can feel each board under foot. I would like to
fill the gaps with something but, as it's a big job that think could
ruin an otherwise very good floor, I would like your advice.

If it bothered me enough to do anything about it I'd be inclined to
go with clear silicone sealant, and see how it lasted.
Or lift the boards and re-lay them.