Is that a tongue-and-groove flooring? Is it only one crack you have noticed? If so, Osmo filler may be a good idea, as solution - I would guess your floors are oiled?
- If the crack was there and the floors are T&G, replacement of a plank locally is possible without affecting other planks.
- If the crack has come up after the floor fitting, it will be advisable to leave the floors for a month without any actions and monitor if more cracks will come up. This maybe caused by other factors, beyond fitting.
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The filler is applied with the help of builders wipes, which clears the excess and removes traces of the filler on surface. You don't have to do anything further.
Osmo filler is coming in few shades - oak, antique oak, white, etc. You can order the product from a sepcialists flooring shop.