For previously varnished floors it is recommended to sand off all previous applications and start from bare surface.
Partial varnishing is not recommended
Remove all previous varnish. Sand the floor and remove the dust.
Fill the gaps with GAP Parquet putty. Prime 3 x with water-borne TOTAL Parquet primer. Intersand between coats. Topcoat 2-3 x with water-borne 1-component OPAL 20 Parquet lacquer.