How To Sand Hardwood Floors

When your hardwood floors need refinishing—and they eventually will—you can save a lot by doing the work yourself. Many homeowners choose the DIY route, which is why virtually every tool rental outlet in the country stocks floor sanding equipment. Before you jump into the messy and physically demanding project of floor sanding, however, you need to carefully inspect your floor to ensure it's a good candidate for sanding. It may not be. If it is, make sure you have everything you need on hand before you start so you can avoid interrupting your work flow with trips to the hardware store. You'll need to rent two pieces of heavy equipment: a drum or orbital sander and an edger. You'll also need sandpaper, scrapers and a few other items, including floor filler.

You need to know what kind of flooring boards you have before you begin sanding, or you could have a disaster on your hands. If the floor surface is hardwood, that doesn't mean the boards are solid hardwood. They could be engineered, which means they are mostly plywood or softwood with a hardwood veneer. Veneers can be extremely thin—sometimes not much more than 1/8 inch ( 2 -3 mm)—and if you sand such a thin veneer, you can easily wear right through it.

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Sanding a solid hardwood floor can also be risky if the floor has been sanded before. Solid hardwood boards are 3/4 inches thick, and each has a groove milled in its side that overlaps the tongue of the adjacent board. The groove overlap is typically about 1/4 inch thick. Sanding removes, on average, about 1/16 inch of material, and sometimes more, so you can safely sand a solid hardwood floor two or three times, but no more. If you sand too much, the overlaps wear thin, and they crack and splinter.

Overlaps on softwoods like fir and pine are even more vulnerable to cracking and splintering. You can sand a softwood floor once with a drum sander, but that's about it. When sanding softwood, it's best to avoid using a drum sander altogether and do the job with a flooring orbital sander. It's less aggressive and doesn't remove as much material.

To check for thin overlaps, press down firmly along the seams between boards. You shouldn't feel any give. If you do, sanding will be a waste of time. The overlaps are likely to splinter and break, and you'll probably end up replacing the floor anyway. If the overlaps feel solid, remove the threshold from a doorway or a heating floor register so you can see the cross-section of a board. This tells you whether or not the flooring is engineered and how thick a veneer it has. If the veneer is more than 2 mm thick, you're good to go.

Your inspection of the floor prior to sanding should include checking for damaged boards that need to be replaced. Now is the time to do this. Look also for protruding nails and pound them down to sink the heads at least 1/8 inch into the wood.

You have a choice of tools for sanding the bulk of the floor. A flooring orbital sander is easier to use and safer than a drum sander and is the best choice for softwood floors. However, it isn't as aggressive and won't cut it if you have to do tough work, such as removing heavy finish coats, flattening cupped boards or sanding an old maple floor. Besides a drum sander or orbital sander, you'll need an edger, which is a powerful disk sander for the edges of the floor. Both tools are commonly available at rental outlets, which also stock the sandpaper.

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The edger doesn't reach into corners, and the best way to handle these is with a 2-inch pull scraper. Purchase several new blades so you always have a sharp edge. In lieu of a scraper, you might consider a multi-tool with a triangular sanding pad, but it won't work as fast or as cleanly. When you rent the large sanders, you might also consider renting a flat sander, which is designed to reach under radiators. Other sanding equipment includes a pad sander for occasional use (you might not need it) and a pole sander for a final cleanup. Speaking of cleaning up, don't forget to buy, rent, or borrow a powerful vacuum cleaner with a crevice tool.

A complete sanding job always involves multiple passes with sandpaper of progressively finer grits. The final grit on most hardwood floors is 80, but if you want a super-smooth finish, or you're sanding old maple or birch, which are especially hard and susceptible to noticeable scratches, you'll probably want to finish with 100-grit. Now you know where to finish, but where to start?

The normal sequence of grits is 36-60-80, but it's better to start with a coarser grit if you have to remove many coats of paint or finish or flatten cupped boards. If you start with 24-grit, plan on making four passes, because it's important to progress through the grits in order, or you may leave deep scratches.

You've cleaned the floor and pounded down protruding nails, so now it's time to make the first pass. When using a drum sander, you normally always sand with the grain of the wood—parallel to the direction of the wood fibers. The exception is if you have to cut down cupped boards or remove heavy finish. In that case, a first pass diagonally across the grain, followed by a second pass parallel to the grain using the same grit paper, makes faster progress. But beware, though, that this procedure removes more than the normal amount of wood. If you're using an orbital sander, you don't have to worry about direction, because the sander leaves the same circular scratch marks no matter which direction you go.

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Use the edger to sand the edges of the floor after each pass, using the same grit sanding disk. Clean up the corners with a scraper or multi-tool, then vacuum and you're ready to move onto the next grit. After you've completed your final pass, clean up the circular marks left by the edger by hand sanding with 100-grit paper. Don't use a pad sander for this—it will leave an area that is noticeably smoother than the rest of the floor. It's also a great idea to do a once-over of the entire floor with a pole sander to help smooth out scratch marks.

It's important to fill gouges, gaps between boards and other defects with floor filler. An easy way to do this is to purchase latex filler and thin it to the consistency of honey. Spread it over the entire floor with a rubber grouting float after you've completed the first pass, and let the filler dry. It will come off the surface of the wood during the next pass but will remain in the crevices. This procedure is faster and more thorough than spot filling.

One of the disadvantages of drum sanders is that they can leave chatter marks, which are ridges caused by subtle bouncing. You may not notice them when sanding, but they can be depressingly obvious after you apply the finish. To avoid them, work slowly and keep light but steady downward pressure on the drum. It's best to use a drum sander with a clutch that raises and lowers the drum. If you have one of these, all you have to do is keep hand pressure on the clutch while sanding.