You will want to sand a floor whose finish - whether varnish or oil - is too damaged for a simple restoration. When the existing finish is too worn out and the wood has been directly exposed in several places, it is probably too late for a restoration and everything must therefore be removed in order to apply a new topcoat. Otherwise a simple restoration will give an uneven finish, meaning that aesthetics and durability will both suffer.

Still not sure if your floor needs to be sanded? If that's the case, you can perform a fairly simple test: Spray your hardwood floor in a few highly stressed areas of your floor with water droplets. If the droplets bead on the floor, the wood is not directly exposed. In this case, the finish may need a good cleaning with a professional soap, or an additional coat of varnish or oil, without necessarily having to start from scratch. On the other hand, if they penetrate directly into the wood and darken it quickly, the existing finish is very worn and the wood fibers are exposed. In this case, the floor must be sanded and finished again. Don't worry, however, you've come to the right place!

Before you begin your hardwood floor sanding project, it's important to know what type of floor you are dealing with: Is it an old floor made of wooden planks that have been covered with a carpet? Are the boards in good enough condition to be sanded and finished again? If there was a carpet, were the clamps and other installation accessories removed from the wood? Some old fastening systems tend to break the boards when you try removing them. Additionally, removing those old fasteners from the planks can also oxidize the wood once they've been removed. Is the floor solid wood or engineered wood? Some engineered floors can be sanded and refinished, but not all, as some are laminate, and cannot be sanded. Better to make sure before renting your sanding equipment for the weekend!

Even when dealing with a traditional hardwood floor, it should be borne in mind that these floors also have physical limitations. Hardwood flooring is usually 3/4 of an inch thick, but the interlocking grooves and tongues are in the center of the planks. Therefore, after three or four complete sandings, a large amount of material will have been removed, and you will be getting dangerously close to those tabs... and nails. In short, when a floor has already been sanded 3 or 4 times, it is better to consider a finish that is very resistant or easy to restore without complete sanding, because if this finish must be replaced again, the entire floor will have to be replaced because you'll have run out of wood to finish!

The number of times a floor can be sanded ultimately depends on the skills of the person sanding the floor, the type of equipment used, the remaining thickness of the "usable" wood layer, as well as the flatness of the floor surface, (ie its level). For example, if a floor has moved or warped over time and is not perfectly flat, more material will need to be removed where the floor is not level.