There are two industry recognised methods for gap filling.
The first is to mix the sanding sawdust with resin to form a paste and trowel apply it across the floor prior to the final sanding. This method is best suited for filling small gaps, <
3mm, in parquet and block flooring. It is a product which is brittle when dry and therefore prone to crack and pop out when used to fill large gaps between strip floorboards.
The second and best method for filling gaps between floorboards is to glue and hammer wedge shaped slivers of pine between the boards and then sand them flush with the floor. This will give the
longest lasting and best looking results as the pine strips will expand and contract along with the floor.