New floors:
Sand the surface with medium grit sandpaper in the direction of the wood grain to remove any imperfections.
Cover the fissures using a mixture of sawdust with glue or mastic suitable for the purpose.
After the fissures have dried, sand it again to even out the surface.
For previously varnished floors:
If the floor is in good condition, wash it and remove traces of fats caused by cleaning and maintenance.
Sand evenly with fine grit sandpaper in the direction of the wood grain to remove the brightness of the previous layer of varnish.
If the surface is in poor condition or waxed, completely remove the wax or varnish beforehand with the aid of coarse or medium sand paper, continue sanding with a medium grain so as to remove or minimize any possible stains or mold contamination.
After sanding:
First vacuum and then wipe down with a damp cloth so as to completely remove the sanding dust.
Let the surface dry completely before varnishing.