Fill gaps with fiberglass?

Q: I live in Singapore and my house has hardwood floor strips (Balau wood from Malaysia). One year after installation there are visible gaps between the floor strips (tongue and grove) and the putty used to cover them keeps coming out when vacuuming the floor.

The contractor suggested we fill the gaps with fiberglass and re-sand after staining: is this practical? I’ve see you suggest a different kind of wood filler, but not fiberglass. Would fiberglass be a good solution? Another contractor suggested to use the same filler as I have today (probably just sawdust and glue), as he says the wood has already shrunk to the max, and it won’t come out… Can I trust him?

A: If you are sure these gaps will not close up with increased humidity, then you need something that will fill the gaps and also accept stain. I don’t see fiber glass doing that. You would be better off using a product such as I found here. A moisture cure adhesive in a bottle which accepts stain, or take a look at Timbermate: