Wooden floorboards can really add character and a natural look to traditional bathrooms, but they do need to be finished properly to prevent water damage.  Depending on the condition of your floorboards, it might be more economical, not to mention quicker, to replace old or damaged boards with new ones, as there is a lot of work involved in preparing floorboards.

Old bathroom floorboards will need to be stripped first of all, to prepare the surface for the new finish.  It's a good idea to hire electric sanding machines for the day to get this job done efficiently.  Generally, you'll need a large drum sander for the large open floor, an edging sander to do the edges, and a corner sander for the corners.  Using a sander is a dusty job, so make sure you wear a dust mask and goggles.  Block the gap under the bathroom door to stop dust getting all over the house, and open the bathroom window for ventilation. 

With a drum sander, you will need to attach the sandpaper to the revolving drum or cylinder and hold it in place with the retaining bar.  You'll need to replace the sandpaper at intervals, as it gets used up.  As a general rule, start off with a coarse grade sandpaper, before using a finer grade to achieve a smooth finish.  Use your sander at 45 degrees to the floorboards.  When starting it, tilt it back, as if you start it flat on the floor you risk scarring the wood.  Repeat the process going at 45 degrees in the opposite direction.  Then follow the direction of the grain.

Once you've done the main central part of the floor, use the edging sander around the skirting board and the base of your bathroom fixtures such as your sink pedestal and toilet.  Edging sanders use an orbital action, so they are good for finishing off difficult areas and for slight depressions in the floor.  Finally, use the corner sander to get into those corners that the other sanders can't reach.  Corner sanders are small and handheld, giving your more control over these tricky bits.

The aim of sanding is to strip off all previous coatings of stain, paint or varnish, to get back to the bare wood and achieve a smooth finish.  This is important as you'll be walking on your bathroom floor with bare feet.  Once the tiring and messy preparation work has been completed, you can apply several coats of your finish - stain, paint or varnish - to create a watertight and attractive surface.  Begin by using a vacuum cleaner to get rid of all the dust on and between the floorboards.  Wipe the floor with a cloth soaked in white spirit to remove the last of the dust.  Then apply your finish with a paintbrush, following the natural grain of the wood.  You will need to apply several coats, so it's worth using an acrylic-based paint, stain or varnish, which will dry quickly, so you can do more than one coat in a day.