One of the most reliable and traditional methods is to mix fine sawdust—ideally from the same floorboards you're working on—with either PVA glue or a suitable resin. Blend the mixture until it reaches a peanut butter-like consistency, then apply it into the gaps using a filling knife. This technique allows the filler to closely match the colour and texture of the floor, creating a smooth, natural-looking finish that bonds well and holds over time.