Can spaces between hardwood floor boards be filled?
I agree, putting a wood filler is not a viable option. The filler breaks down over time and you're left with the same gap as before. I think gaps of between 1/16-1/8" are a character trait of an old hardwood floor and usually look very good if sanded and finished well. However, gaps of 1/4" or more are usually caused, or at least in part, by uneven joists and sub-flooring. If you don't like the look of the gaps aka "old wood flooring", your best option is to replace the floor boards. I tend to like the look of a old floor, gaps and all!