How to Sand a Deck with a Random Orbital Deck Sander

All exterior wooden structures have one thing in common: They all require maintenance.

This is certainly the case for wooden decks. You’ll need to work to maintain your deck, and you may eventually even decide to fully restore it. The first step in this process is sanding it with a random orbital sander.

A random orbital sander is a specialized tool that sands wood in an elliptical pattern and it’s the most efficient sander to use when refinishing a deck. Random orbital sanders are easy to use and are effective for this task. This is because the abrasive sandpaper motion travels in an irregular pattern instead of a consistent line or circle. You’ll see far less mechanical patterns on your deck surface when you use a random orbital sander than you would with other types.

Every sander performs the same basic task. They use abrasive materials to cut through unwanted material and leave a smooth finish ready for staining and sealing. Understanding how your sander works is an important part of achieving a great deck finish and can help you get the best possible results. It also helps you make sure that you’re always sanding safely.

When starting a deck refinishing project, many people jump to specific details like what grit of sandpaper to use, or whether they need to sand the entire deck before staining. We’ll answer those questions – but first, we’ll review some basic information you need to know before sanding your wooden deck.

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Choices in Sanders

There are a few different sanding methods you can use on your wooden deck. They range from hand-sanding, which is time-consuming and labor intensive, all the way to renting a drum sander that professionals use to finish hardwood floors. We don’t recommend either of those. Hand sanding takes a long time and is work-intensive, while drum sanders are tricky to use and have a long learning curve. They can also get away from you and cause more damage than good.

That narrows your choices down to belt sanders and orbital sanders. Belt sanders are aggressive tools, especially when equipped with coarse sandpaper. They remove deck material fast but can be hard to handle and difficult to work in tight places. Orbital sanders are more user-friendly. They can be operated with one hand and fit into close spaces. They’re also great on handrails.

If you need to sand the handrail or corners of a deck, then you’re on the right track when you choose an orbital sander. But don’t just choose a regular orbital sander that spins in a consistent circle. They work where a belt sander won’t, but straight orbital sanders leave swirl marks because of their consistent motion. You can only vary a regular orbital sander’s motion by moving it by hand and compensating with your own pattern.

That’s not the case with a random orbital sander. The operative word is random. Its design allows it to move in an elliptical manner, which blends sanding directions of with-grain and cross-grain. Random orbital sanders let you work across your entire deck without having to worry about always keeping in a straight line. They make your deck sanding job more even and a lot faster.

Types of Random Orbital Sanders

There are a few different types of random orbital sanders you can choose from for your deck restoration project. Brands can be your personal choice but the type of sander must match your needs. Determining the type of sander starts with selecting the power supply. There are two types to choose from:

  • Pneumatic Random Orbital Sanders are powered by compressed air. They require expensive air compressors and hoses to operate. Pneumatic random orbital sanders are the best choice for auto body repair shops. They’d work on a deck, but purchasing an air-powered sander for a deck project isn’t practical.
  • Electric Random Orbital Sanders are by far the best selection for wooden deck refinishing. Electric sanders operate on 110-volt household current and their amperage requirements vary depending on the size of the motor. You don’t need an expensive support system with an electric random orbital sander and they’re also a much more affordable option.

Once you’ve decided on an electric random orbital sander, you need to consider size. This includes the actual size of the sander, its power capacity and the size of the sanding surface. This determines how easy it’ll be for you to operate the sander as well as how much material it can remove in a given time. For most projects, you’ll want to choose one of two main orbital sander sizes:

Recommended Sanders for Deck Sanding

Electric Orbital Sanders

Pnuematic Orbital Sanders

Grits of Sandpaper

Now that you’ve chosen what random orbital sander works for you, it’s time to select the right coarseness or grit of sandpaper to use on your wooden deck. Sandpaper is rated in numbers and is runs from very coarse at 60 grit all the way up to very fine at 240 grit. Sixty-grit removes material quickly but leaves the surface rough. High-grit paper, on the other hand, doesn’t remove as much but gives an ultra-smooth finish.

What you want is a two-step sanding process. Our recommendation for wooden deck sanding is to start with a coarse paper like 80 grit and give the surface an even pass. Don’t be too aggressive and create dips and gullies, but don’t be too obsessive and expect perfection. Then, give your deck a second sanding with smoother paper like 100 grit.

You shouldn’t need any more than these two sanding grits, but it depends on the type of wood you’re working with. Soft woods with open grains like cedar and fir sand quickly and may need a finer sanding to make sure their pores can be properly sealed. Hardwoods such as teak and oak have closed grains and naturally seal better than open grains. Make sure you select sandpaper grits that work well with the type of wood in your deck.

You should also make sure you buy quality sandpaper from a specialized supplier. There’s a vast difference in the types of sandpaper available today. More expensive sandpaper has superior cutting abilities and will stay sharper much longer than cheaper sandpaper commonly sold by big-box stores. With sandpaper, you get what you pay for and you’ll use less paper in the long run if you start with a better product.

Deck Sanding and Sealing Steps

Now, you’ve got the basics out of the way. You’ve chosen the right size of random orbital sander and you’ve selected the proper sandpaper grits for your deck. Next, it’s time to follow a few steps to get a deck surface that looks like new.

Cleaning your deck is the first step in your refinishing process, but the way you do this depends on several factors. The two main things to consider are the type of wood it’s made of and the finish it currently has, but you’ll also need to consider the amount of grime and dirt that’s accumulated. You might be fine with simply sweeping off and hosing down your deck off before plugging in your sander. But if your deck has built up a layer of dirt and grime, you might have a bigger job ahead of you.

You can clean most wooden decks with a mixture of detergent and water. You can apply this cleaning solution with a stiff deck brush that’s designed for the job, then hose it off with a garden sprayer. Where your surface is oily or has organic buildup, try using trisodium phosphate (TSP) in a hot water pail. TSP is inexpensive and widely available. It does a great job of cutting through tough stuff and leaving a clean surface that’s ready for sanding, sealing and staining.

Larger wood decks with dirty surfaces are good candidates for power washing. Power washers remove dirt fast and with minimal effort. There are a few things to be aware of when using a power washer on a wooden deck, though, particularly a deck built with softwood that has open grain. Make sure you have the pressure set at a lower rate of around 2,000 psi. Higher pressures like 3,500 psi can be too strong and rip the wood fibers open.

Use the fan head on your power washer instead of the direct nozzle and be careful of the distance you’re holding the nozzle from the deck surface. It’s best to start with a test area near the outer edge of your deck and approach it slowly to get a feel for how your wood reacts to being pressure washed. Once you’ve gauged the right pressure, work from the deck’s inner edge towards the outside. It’s better to blow debris and water away from your house than right at it.

Drying Your Deck Before Sanding

After you wash your deck, you’ll need to let it dry before you can sand. The amount of time this takes depends on a few factors, including the type of wood on your deck, the ambient temperature, the relative humidity and whether your deck gets direct sun. Washed decks in the hot sun will dry much faster than shaded ones in cool weather.

As a result, the time elapsed after you clean is much less important than the moisture content in the wood. Sanding a wet deck will only result in clogged sandpaper and a frustrated operator. Ideally, your deck should appear dry to the eye and contain between 15-18% moisture. You can use a moisture meter to check, but an easier method is the Tupperware test. Place a clear Tupperware bowl over a deck board and watch. If condensation forms in the bowl, it’s too wet. If not, you’re ready to start sanding.

Another test of moisture content you’ll immediately notice is dust. If sanding produces wood dust, you’ll know your wood is dry enough to work with. In fact, very dry deck wood can produce so much sanding dust that it can be a problem for you, occupants in your house and even your neighbors. If you know you’ll be working in very dry conditions, you may want to select a random orbital sander with a dust collection system built in.

Deck Sanding Process

It’s best to sand your wood deck following a specific process. The beauty of working with a random orbital sander as opposed to an inline belt sander or an ordinary orbital sander is that you’re not restricted to a particular sanding order. You don’t have to start in one spot and continue in a pattern. But it’s best to start with where your deck connects to your hose and work outward. This process is simply more efficient and less time-consuming.

Once you have your main deck area sanded twice—once with coarse grit sandpaper and the second pass using finer grit—pay attention to tight areas like corners, balustrades and your handrail. Random orbital sanders are excellent for these areas. They’re also quicker and produce a better surface for sealing and staining than hand sanding will.

It’s important to know when to stop sanding, though, as there will be a point where excessive sanding is unnecessary and can remove too much wood. Knowing when to stop is a judgment call but as a rule of thumb, you should stop when the wood pores open and the entire surface is consistently smooth.

Personal Protection

Random orbital sanders are generally safe but you can never be too careful when using power tools. All modern sanders come equipped with devices to prevent electrical shock and many are designed with ergonomics in mind and minimize the threat from vibration. But there are other personal safety concerns you should pay attention to when using a random orbital sander on your deck. Consider using these pieces of personal protection equipment:

  • Eye protection
  • Hearing guards
  • Heavy gloves
  • Knee pads
  • Respirator or dust mask

Another safety concern is your physical condition. Sanding a deck is a long and laborious process and can be physically demanding. Consider if you’re physically prepared to do all your sanding in one continuous effort or if you should break it into more manageable time slots.

You should also be sure that you’re purchasing your sander from a reputable supplier. National Abrasives, Inc. is a family owned and operated company that’s supplied random orbital sanders and other abrasive equipment for over 20 years. We carry a wide range of sanders from top-quality manufacturers like Mirka, including DEROS dual action sanders, pneumonic random orbital finishing sanders and DEROS electric sanders. We also supply quality sandpaper that’s the right grit for your wooden deck.
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*Updated February 20, 2020

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