As with any painting or varnishing job, preparation is the key. However, the correct preparation is even more essential when the paint or varnish is being applied to floors. Floors obviously take more of a beating than walls, etc, so getting it right first time will mean less repair work in the future.
First and foremost, clear the room of ALL furniture, etc. Now vacuum the room from edge to edge (in fact especially the edges). Once the floor is clean, use a lint-free cloth dipped in white spirit to further clean the boards. If you want to bring up the grain of the wood slightly, take a wire brush and carefully stroke it along each board following the direction of the grain.
If the floor is already painted or varnished, rub it down well with abrasive paper to strip the boards back to bare wood. This is backbreaking work, so hiring a floor sander might be a better option, especially for big rooms. Floor sanders are very powerful, noisy and create a lot of dust, so take your time and make sure the room is well ventilated. You can also hire a smaller sander designed to let you do the edges of the room without taking gouges out of the skirting board. Using a floor sander will result in a much smoother finish than sanding by hand.
Wood stain can create a very attractive finish and is available in a wide range of colours. Wood stain will also act as a protective coat. Wood stain is available as a water-based and a solvent-based product. The water-based wood stain dries much quicker than the solvent-based, but will not give quite as protective a finish. Seal your new (or newly stripped) floor with a thinned Polyurethane varnish before you start and allow it to dry. Wood stain may require up to three coats, so make sure you buy enough to start with. As with any flooring project, make sure you work towards the door and not into a corner. You might be stuck there for some time!
Limed boards, if done correctly, can look fantastic. You can even apply it over a colourwash to create an effect sometimes called "pickling". To apply the liming paste, you first need to strip the boards back to bare wood (if they are already treated). Once you have done this, gently stroke the floorboards along the grain with a wire brush and paint with a coating of Shellac to seal the wood. Once the Shellac is dry, take fine wire wool and apply the liming paste. Work into the grain in small circular movements until the whole area is well covered. Leave the paste to dry thoroughly before applying some clear paste wax. This will help to remove any excess liming paste as well as adding another protective coat.
You can buy specially formulated floor paint, which give a very durable finish. However, they do cost more than standard paint, and in all but the highest traffic areas, standard emulsion paint with a coat or two of varnish will do the job. Painting your floorboards tends to be a much quicker job than varnishing or staining as you can paint over existing coverings. Just make sure it is free from dust and grease, and rub it down slightly with abrasive paper. Be aware that painting your floor will hide a lot of the grain.
Satinwood paint can give a really nice finish to bare floorboards. Choose your colour (anything from blue to pale green can work well on floors, so it is worth experimenting in a small area to find a colour you like) and dilute it with 50% white spirit. With a wide brush, apply the wash in the direction of the grain and then, while it is still wet, wipe it down with a cloth. This will reveal the higher parts of the grain in a lighter shade. Once the wash is dry, coat with a clear varnish.
Like wood stain, Floor Varnish is available in either water-based or solvent-based formulas. Solvent-based offers the best results in many cases, but can be unpleasant to apply due to the smell. Varnishing your floorboards can produce a stunning finish, and there are many shades to choose from. However, even with the three coats that you will need to apply, varnish will inevitably begin to crack after 3-4 years. Fixing this will mean stripping the floor back to bare wood and re-varnishing. As with all floor treatments, work towards the door, not away from it!