First take off the existing floor treatment and remove any protruding staples, nails, or splinters. Fill any large holes with wood filler, then sand. If you have any really rough areas give them a good sanding too. Then clean, clean, clean your floor. Sweep, vacuum, then sweep again to get rid of all dirt and dust bunnies.
Next, use a good primer. Our home was previously a rental and there were odd stains left on the floor. Primer prevents these stains from bleeding up through the new paint and, if they're pet stains, it blocks odors too. Primer helps paint to bond better to the existing surface, not to mention, you use less paint when you use a primer.
Our sub-floors are particle board on top of plywood. They look like this sample: As I said, they were not in the best of shape. There were some areas that "blistered" because of old spills. You can still see those areas, but you really have to look. The paint helps disguise the imperfections. Speaking of imperfections, I did not do anything with the areas where the boards butt up against each other. I looked into filling the crevasses but came to the conclusion that it would be a lot of work and may later become troublesome because floors are always moving; expanding and contracting with temperature and traffic conditions. Whatever you fill the cracks with will also have to expand or it will crack or peel. There are some caulks made to stretch but I chose not to spend the money.If you are lucky enough to have plywood or pine floors, hooray for you! If you're worried about splinters remember that paint acts as a bonding agent. As long as you sand down any worrisome areas before painting you shouldn't have any problems. Another nice thing about paint is that you can easily go back and touch up any gouges or wear spots.
I used two techniques on my floors. On our stairs and in our bedroom I simply painted with a latex paint. I did two coats. As you can see, I laid rugs in the high traffic areas.
I'm very pleased with the way both areas look.In my entry, hall and family room I put in more work. These areas get A LOT of wear! Plus the entry needed to stand up to wet shoes and puddles of melting snow. I primed them and then painted two coats of latex paint. I used a roller with a long handle attachment.
*HINT: Be sure that you plan out a painting strategy---you don't want to literally paint yourself into a corner.*
After the glaze dries thoroughly, roll on a couple of layers of a good polyurethane to protect the floor from wear and moisture as well as give it a little shine and make it easier to clean and maintain.It's a time consuming process; taking a few days. But it is a lot less expensive than carpet or any other flooring. It's also a good way to test out what it's like to live with a bare floor before you make the investment into hardwood or laminate.