How to Sand a Deck Before Applying a Finish

When you’re preparing a wooden deck for a new finish, it’s a good idea to scrub or power wash the surface first. But, this process may cause the wood fibers to swell and/or lift away from the wooden frame surfaces. After drying, this can lead to the formation of splinters and the deck may be raised. Learning how to sand a deck with a light, but thorough sanding can prepare the surface for sealing and staining.

What Kind Of Sander Should I Use To Sand My Deck?

When you’re exploring options on how to sand a deck efficiently, it’s easy to become overwhelmed. There are many power sanders to consider from smaller orbital models up to commercial sanders that you can lease from rental outlets or home improvement stores.

All these tools can work well, but they must be used properly, and using hand sanders will extend the work time considerably. If you need to sand a larger deck, it may take several days to complete the sanding with a hand held orbital sander. Most wooden decks are made from softwood, and these materials are prone to damage from belt sanders.

So, a good orbital sander is usually the best choice, but you need to be patient and methodical to get the smoothest surfaces.

Should I Sand Or Clean My Deck First?

Before any products are applied to a deck surface, they should be thoroughly cleaned and sanded. Power washing should be followed with 24 or 48 hours of drying time before sanding commences. If there is any rainfall during that drying time, you will need to start the drying time again and it may be advisable to cover the deck in tarps to protect it.

The deck can be cleaned with mild detergent or deck cleaning products. You can use a power washer, garden hose, or bucket and sponges to get the job done. If you have an older wooden deck, you may need anti-mildew treatments and a paint or stain remover. The exposed deck board ends and visible joist surfaces should be thoroughly cleaned and these areas are easy to miss.

In order to balance the pH levels and restore the luster, you may want to apply a wood brightener product when the cleaning is complete.

When the wood is fully dry, it’s time to sand.

What Is The Best Way to Sand A Wood Deck? 

When you are learning how to sand a deck, it’s important to understand that this stage may not be strictly necessary, but it will always improve the finish. When the deck is sanded, the previous layers of stain and paint are stripped away and the colors of the wood are more even.

This will make any fresh stain or paint re-coating easier and more even. When the splinters and damaged wood sections are smoother the final results will be improved. Here are three simple steps to follow:

1.    Inspect the Deck

Take a walk around the deck and check for imperfections, such as broken wood, warped timber, raised nails, and more. Remove or replace these imperfections and hammer down raised nails with a countersink. This will preserve the sandpaper for longer and make the sanding process much easier.

2.    Sanding the Deck

When you’re learning how to sand a wood deck, it’s important to consider your safety. Always wear PPE to protect against breathing in dust and getting debris in your eyes.

If you’re using a belt sander on splintered areas to remove a pre-existing stain, it’s a good idea to start with coarse 40-grit sandpaper. Move the belt sander forwards and back again along the board grain and gradually switch to finer grades of sandpaper.

80-grit sandpaper will give your wooden boards a very smooth finish. Using an orbital sander will take longer, but the risk of gouging the boards is reduced and an edger sander can be used in the corners.

3.    Sanding Deck Railings

To get the best results use 80 or 100-grit sandpaper to sand the crevices and surfaces carefully. The railings are usually the most visible part of the deck, and they are the surfaces that are most likely to be touched by hand.

So, they need to be properly cleaned and any holes, cracks, or scratches should be filled with epoxy wood filler. After the filler has had time to cure, sand the railing surfaces and you can use 20 grit on the uprights which are not touched to save time.

It’s important to avoid over-sanding these areas because this will prevent full penetration into the wood when the stain is applied. Before a stain or other product is applied, it’s essential to wipe, vacuum, and clean away any dust.

How to Sand a Deck with Grooved Decking

It’s a real challenge to sand between deck boards because the gaps may be irregular due to wood shrinkage. Switch to a smaller detail power sander or a sanding block with 60 or 80 grit sandpaper to sand both of the gap sides at the same time. Rounded deck board edges should be sanded with a rotation of the sander after each pass.

Tighter gaps can be sanded with a stick covered with sandpaper and a sawing action. Even sanding on visible surfaces and between boards will prepare the boards for an even application of stains, paints, and other products.

How to Sand a Deck –FAQ’s

What if it rains after sanding the deck?

After washing, the wood should dry for at least two days for the best sanding stage which comes next. But, if there is a chance of rain, it’s a good idea to protect the deck to maintain or continue the drying process. If the deck does get wet before or after sanding, the drying process must be repeated before any products are applied.

How long after sanding can I stain?

After the deck has been thoroughly washed and sanded, it may be possible to apply products after a 24-hour drying period if the weather conditions are especially dry.

Do I have to wash the deck after sanding?

Yes, this is a crucial step in how to sand a deck before applying a finish. The deck must be cleaned to remove the wood dust and other sanding debris from the surfaces and the wood pores. But, if you power wash a second time, the wood fibers can be raised again and more sanding would be required.

So, the second cleaning after sanding should be restricted to vacuuming, wiping, and sweeping. A tack cloth is ideal when you need to wipe areas of the deck that are harder to reach.

Can I sand a damp deck?

No, the wood must be fully dry and brightened before sanding commences. Certain cleaning products may require longer drying times and the instructions should be followed carefully. To test the wood, sprinkle some water on the surface and if it absorbs quickly, the deck is ready for the next stage.

How do you know when you’ve sanded enough?

To avoid over-sanding, draw a light pencil line on the wood and when it’s sanded away, it’s time to move to a smoother grit. Repeat this process a few times up to the highest grit of sandpaper that you want to use. Then, wet the wood with mineral spirits to find any remaining marks that require a final sanding pass.

Why is my wood not smooth after sanding?

The sanded surfaces may look good at a distance and they may even feel pretty smooth to the touch. But, when the stain or finish is applied, the presence of sanding scratches will be evident.

Any sanding mistakes must be fixed before they become embedded in a layer of dyes, resins, and pigments that will accentuate them. A final sanding pass after the aforementioned application of mineral spirits can make a dramatic difference to the surface.

Why is my wood green after sanding?

The usual cause is a deep old wood stain in the grain that the stripping agent could not penetrate. Sanding may not remove this discoloration because the deck grooves can be too deep to reach.

What happens if you sand too much?

If you sand up to and beyond 240 grit sandpaper, you will have fine wood dust that clogs the wood pores. This will prevent the penetration of the stain or finish and the final finish may be poor. Over-sanding in an isolated area can create wood divots which will look strange when the finish is applied.

Can you over sand wood before staining?

Yes, over-sanding creates uneven surfaces which are common in places where an attempt to fix a defect, gouge, or scratch is attempted.

How to Sand a Deck – In Conclusion

Learning how to sand a deck before applying a finish will create a smooth and absorbent surface that will improve the final finish. At least two or three passes with a power sander using 20 or 50 grit followed by 60 grit and up to 80 or 100 grit is a tried and tested approach.

Each sanding pass will take around 5 hours on a 100 sq. ft. deck and the surface needs to be dry for each pass. If it does rain between passes, there should be a two-day drying period before sanding recommences.