As a general rule, a finish that was done prior to 1930 is probably an old tung oil finish or shellac that by now has many coats of paste wax on it. You can clean this floor with odorless mineral spirits. Use a soft white floor cleaning pad to dissolve the old wax into the solvent and while still wet wipe it up with clean rags. Wait over night for the solvent to dry and wax with a liquid paste wax. There are two available; Lite n Natural by Bruce, or try Dura Seal’s Liquid Floor Wax. These waxes have the same consistency of semi melted butter. That makes them easy to apply, and be sure to do a thin coat. It will haze over when it’s dry (about 30 minutes), and then you can buff it with an electric buffing machine. Try to find one that has natural bristle pads on it, this will easily buff the floor to a nice satin sheen. You can also use the paste waxes from those little tins. The best method for these is to wrap a golf ball sized lump of wax into a piece of cheese cloth. Squeeze it gently and apply a thin coat to the floor and buff it when it hazes.
A common finish used in the 1940’s and beyond would be floor varnishes and lacquers and later polyurethane. I have managed to test and recoat even 50-year-old finishes. Just be sure to do an adhesion test, before you commit to recoat the whole floor. A poorly adhered floor finish will scratch with the least provocation. Once it starts peeling you will have a mess on your hands until you resand and refinish the whole floor. Prepare a small area, by scuff-sanding and cleaning, and apply the proper matching finish. Old alkyd varnishes can be recoated with modern polyurethane, as they are both classified as reactive type finishes. Lacquer, shellac, and water based are best recoated with the same type of the finish test spot have cured, and that depends on the finish. Water based and lacquer finishes take 2 weeks, and oil based urethanes up to a month. If you’re pressed for time, wait at least half that time. Then using a razor, cross hatch the spot and apply a piece of duct tape. Rip it off and take a close look at the test spot. If you see any missing finish or edge peeling at all, don’t recoat the floor with this method. You will most likely need to resand and refinish instead.
A lot of people are using the newer water based finishes, and as long as they are one of the two types I mentioned before, you should get up to a decade of use out of them. These finishes together with the finishes already on most prefinished floors are the only ones that should be recoated with the new recoat-without-resand kits, that I mentioned in the first part of this article. I find it quite disturbing to see some of these recoat kits that claim that they are good for recoating any kind of floor finish. They usually include a rather less durable water based finish, which most people find will scratch even easier. These products just keep you coming back for more. If you really want to use this method, try one of the pro kits I mentioned before.
So, like most things, it takes a little knowledge to find the right maintenance products and methods to suit your floor needs. A well maintained 3/4″ strip floor can be screened (scuff sanded) and recoated every 10 years, and if done 3 times you should be able to make the original floor sanding job last 40 years. You could then resand the floor 6-8 more times, giving this floor at least a 300-year life span. But you can do a similar treatment using acrylic wax. Just strip and re-wax once a year. And only resand and refinish every 40 years or so. You can avoid all those nasty fumes of recoating, and your floor will always have a high gloss. If you want, experiment with those refresher or re-coat kits offered by companies like Bona Kemi or Basic Coatings. Take your choice. It’s your floor.