This is a somewhat tedious job, but it is a great way of restoring a very old floor whilst retaining all of its character.
Use a stiff bristled scrubbing brush and soapy water to work a small area at a time. As you clean each area, mop up the dirty residue and rinse with clean water. You will find that most old floors require repeated scrubbing to release all the build up of years of grime. Steel pan scourers are also very useful to work with since they have quite a coarse action.
You will need to wear protective gloves and should be careful to guard against splinters. Once the floor has been thoroughly cleaned, allow to dry completely before giving the whole surface a very light rub down with medium grade sandpaper. Remember to wear a suitable dust mask. This is a way of removing any slightly rough or sharp edges, but care must be taken not to generate lighter areas or a patchy effect will result.
Vacuum to remove as much of the dust as possible.
When varnishing a floor, allow plenty of ventilation to prevent build up of fumes and check the manufacturers instruction regarding safety.
A little planning will save you getting stuck in a corner waiting for it to dry. Work from the far side of the room back towards the door. Using a decent size brush – say 4", varnish the boards lengthways.
The first coat will need to be thinned to act as a sealer. If you fail to do this, you will find several coats will be completely absorbed into the surface. Thinning instructions for this first coat will be detailed on the tin.
Apply the varnish in the direction of the grain to a couple of boards at a time along their entire length. Try to keep the edge between these and the next reasonably tidy to avoid "doubling up". Continue across the room and then allow to dry completely.
Apply at least two further full coats (unless otherwise directed) allowing each to dry.
For a really good job, lightly hand sand with a fine grade sandpaper before the final coat. Remove the dust by vacuuming and wipe the surface with a tack rag – a special oily cloth designed to pick up all traces of dust from the surface.