When your deck looks the worse for wear, you can often give it new life by power washing and restaining. Although this treatment will remove flaking finish and the gray surface layer from old softwood and hardwood, it won't smooth out cupped boards, and it leaves the wood with a fuzzy, splintered appearance. If you really want your deck to look like new, go ahead and rent a flooring orbital sander. Some purists recommend against using flooring equipment on decking, but they probably haven't tried to sand a 200-square-foot deck with a handheld belt sander and palm sander. These tools are slower, require vastly more effort and don't do as good a job as the heavy-duty tools made for sanding large horizontal surfaces.
Use an Orbital Sander—Not a Drum Sander
A flooring drum sander removes too much material too quickly to use safely on a deck. Most decks are made from softwoods, and the surface is more uneven than an interior floor. Every time you run over a bump on a deck with a drum sander, you're likely to create a deep gouge. The same is true when using a flooring edger, which is a heavy-duty disk sander designed to remove finish from the edges of the floor and smooth out the wood. When sanding a deck, it's best to avoid both of these tools.
Instead of a drum sander or edger, the best tool for sanding a deck is a flooring orbital sander. This is essentially a heavier (50 pounds or more) version of a handheld orbital sander, equipped with a handle that you use to guide the tool over the surface you're sanding. Outlets that rent flooring equipment usually stock these sanders, and they also sell the sandpaper you need. The typical sanding regimen is to start with 36-grit paper, progress to 50- or 60-grit and finish with 80-grit, but if your deck is in poor shape, you may want to start with a coarser 20-grit sanding disc.
Besides the flooring sander, you'll also want to have a handheld belt sander on hand for working down raised sections of the deck and a palm sander for sanding corners. You might also need a pull scraper to remove tough finish from corners and from under railings. Don't forget to bring a hammer, nail set, drill and screw bit to the job. Virtually every old deck has raised nail or screw heads that need to be reset before sanding.
Things You'll Need
Flooring orbital sander
Sanding discs (36-, 60- and 80-grit)
Handheld belt sander with sanding belts
Drill with screwdriver bits
Epoxy wood filler
Wood sealer or stain
Start by Power Washing
Even if you've decided that power washing won't be enough to restore your deck, you shouldn't skip this step. Power washing removes old finish, making sanding easier and more productive, and it washes debris from between the decking boards. It also removes mold and sundry oily deposits that can clog up sandpaper.
After a thorough power washing, you should wait at least 48 hours for the wood to dry before sanding. Once the wood is dry, the deck may look better than you thought it would, and at that point, you may even decide to skip the sanding. If not, check the weather report to be sure there's no rain in the forecast for the next few days before carrying on.
Just Before Sanding
Before breaking out the sander, examine the entire deck carefully for raised nails or screws and "set" each one you find so it is recessed at least 1/4 inch below the surface. You may come across nails that have are loose enough to pull out with your fingers, Replace each of these with a 3-inch decking screw driven close to the original nail hole, but not directly on it.
Use epoxy wood filler to patch large holes and crack, and wait for it to set up and fully harden before sanding. This type of filler hardens in the wood and can then be sanded to a smooth finish. Spread it with a putty knife and scrape off as much excess as you can while it's still soft. Avoid ordinary exterior caulks and fillers that don't harden, since they won't bear up under sanding.
Sand and Seal
Sanding with an orbital sander isn't direction dependent. The final appearance of the wood will be the same whether you move the sander with the grain of the wood or against it, although going with the grain of the wood is generally more efficient and cleaner. Go over the entire deck with each sandpaper grit before moving on to the next one. If you need to clean up the edges with a hand sander, use the same grit paper you used with the orbital. Scrape hard-to-remove finish using a pull scraper.
After sanding with the finest grit paper in your regimen, vacuum the sanding dust or sweep it off the deck. Don't get the wood wet, because water will raise the wood grain and create the kind of furring you are trying to eliminate through sanding. Apply at least one coat of sealer—whether it's stain, varnish or a clear finish—immediately after sanding to avoid having the freshly sanded wood collect moisture from the air. You can take your time applying additional coats, but it's important to get that first one onto the wood as soon as possible.