The Deck Handbook: Part 10 Staining, Sealing & Maintaining Your Deck

Wood rots. Some woods rot quickly, some rot very slowly but all woods exposed to the elements are going to deteriorate to uselessness over time. Some composite decks also rot. Well, not rot exactly but they do deteriorate. They're not supposed to, after all, they're supposed to be "lifetime" decks but historical evidence shows that most will. It usually takes longer for decay to set in, and it manifests itself in ways that you won't find in real wood but, eventually, deterioration does begin, and has to be dealt with.

The culprits are the sun and water. Water seeps into the wood and provides a splendid environment for the mold, mildew, and bacteria that consider wood to be better than Ho-Hos as a yummy snack. Even pressure-treated wood will eventually succumb. Treated lumber is not sealed against water; it is merely dosed with an unpleasant chemical soup that kills mold, mildew, and bacteria — for a time. The treatment is just skin deep, and it gradually loses its effectiveness over time, which is why you should expect only a 15-year lifespan from an unsealed treated pine deck. Sealed and maintained, however, and a 30-year lifespan is more common.

Unprotected wood also fades and turns gray. Some of this discoloration is caused by the action of mold, mildew, and bacteria in the wood but most is caused by ultraviolet (UV) radiation from the sun. Visible light, the wavelengths we see with, cause some fading, as do the long infrared waves that we feel as heat. But, their contribution is minor. By far the major players are the short UV rays that comprise just 2% of the sun's total energy output but cause 98% of the damage.

Keeping Water Out

Keeping water out of wood is the job of sealants. For decks, the usual sealant is a penetrating solution rather than a surface coating like paint. In the past penetrating sealants lasted, with luck, for a full year. Every spring the deck has to be sealed again. Not an onerous task but irritating and messy.

Today's sealants are much, much better. Most paint manufacturers have deck sealants guaranteed to last 5 to 10 years or more. Many are now offering "lifetime" multi-layered sealants that never need to be reapplied and extend the life of a pine deck to 30, even 40 years. None of these is terribly expensive. Most of these products can be used by a handy do-it-yourselfer. Once the deck is properly sealed, it does not need to be resealed for a long time; but take the "lifetime" claim with a big grain of salt.

Blocking Ultraviolet

So modern sealants have pretty much licked the water problem. The ultraviolet problem is another story.

No one has yet developed a reliable long-term anti-UV coating. Most last just a few years. Ultraviolet is the solar death ray that kills decks — causing deck wood to fade and turn gray. Keeping UV from reaching the wood is the second important sealant function and right now it has not progressed as far as the water barrier function.

Consequently, while we are pretty sure that lifetime sealants do indeed protect a deck from water penetration for a long, long time, we see fading and graying after 5 years. This is not good because once graying starts, the only way to fix it is to sand away the old sealant and grayed wood until fresh wood is revealed, then stain and reseal. And, what use is a lifetime sealant if you have to remove it every few years to restain?

So, we pretty much stick with 5-year coatings and renew the coating before any graying occurs. This keeps the deck looking new(ish) for a very long time. How long? We are not sure yet. None of us has lived that long. Some decks built over 15 years ago (when treated pine was still treated with arsenic) still look relatively new after multiple resealing. We suspect that the new ACQ-treated pine if properly resealed every 5 years or so, is a lifetime product. But, that's merely a suspicion — none of us have lived that long — yet.

Composite decks need to be sealed. Composite manufacturers steadfastly refuse to call the sealing products they use "sealants" — because most advertise their products as never needing sealing. Nonetheless, most composite manufacturers, while stopping short of recommending periodic coating, suggest that treating the deck every year or so with a "protective coating" to guard against stains, mold, and mildew would do no harm.

If this sounds like this "protective coating" might be a sealant, you're right, it is. There are many brands of composite sealants, so use the one recommended by the manufacturer of your deck. If you have lost the instructions that came with your decking material, go to the manufacturer's website or call its helpline to find out what "protective coating" is recommended. And, remember, it's not a "sealant", wink, wink!

Killing Mold and Mildew

A good sealant keeps water out of deck wood, and the absence of water inhibits the growth of mold and mildew. But, the microbes are not eliminated, just restrained. Some new coatings include an anti-microbial agent. The best known is Microban®, a tradename for a collection of anti-microbial products, which is showing up in everything from tile grout to pet beds and computer mouse pads. The various active ingredients in the product are supposed to be harmless to people and pets but genocidal to mold, fungus, mildew, and the rest of the microbial family.

None of the anti-microbial agents lasts forever but can be renewed each time you reseal your deck. We don't think these are strictly necessary in a good sealant that performs its basic function of keeping water out. But, they do no harm, and, who knows, they might even help by summarily assassinating the occasional pesky microbe that slips in despite all precautions.

Adding Color

Most sealants contain small particles of pigment suspended in the solution (or that will be suspended once you stir it thoroughly to the manufacturer's specifications — hint, hint.) These add color to the deck.

The pigment particles collect in the porous parts of the wood, the part we call "grain" and make it a little darker than the rest of the wood that is less porous and absorbs less pigment. This helps contrast the wood grain as well as even out the color of the deck. In many cases, stains are used to change the appearance of the natural wood, for example, making pine look more like redwood or cedar. But, it is also common to enhance the "natural" color of the deck. Most natural mahogany when new, for example, is bland, with only the slightest reddish tinge. It takes 100 years or so for mahogany to develop a deep reddish color. The red we have come to associate with mahogany is usually a stain.

Stain works relatively well in sealants. There is some interference with permeability since the relatively large pigment particles are not at all permeable. But, in most cases, this has minimal impact on the effectiveness of the sealant and the enhanced appearance of the deck makes the trade-off well worth it. Some professionals resist stain/sealant combinations, preferring to stain and seal in separate coats. We think this is more work than it's worth.

Routine Deck Maintenance

All decks require maintenance if they are going to last. There is no such thing as maintenance-free decking. Even our (Almost) No Maintenance Deck is merely "almost" no maintenance.

Decks need to be recoated less often these days. The power washing and sealing every spring is a thing of the past. Some new wood sealants last five years and more. But, every deck needs to be, at minimum, inspected and thoroughly washed every year — just washed, not power washed. So, it's best to understand what your deck is made of and the maintenance schedule you should follow for a long-lasting deck. Here is the process we follow to clean and reseal a deck.

Step 1: Inspection and Repair

Visually inspect the deck. Look for loose fasteners. Nails have a bad habit of working loose in wood over the course of a year. Wood shrinks and swells, loosening the nails. Set all loose nails. It a nail does not appear to grip any longer, remove it and drive in a new nail about 1" away.

We build our decks exclusively with triple-coated corrosion-resistant screws designed especially for the outdoor environment of decks. These rarely work loose but if you see a screw head sticking up, retighten the screw. If the screw won't tighten, there's an old carpenter's trick for that. Squirt in a dose of Behlen Swel-Lock. It will swell the wood fibers permanently, which gives the screw a new "grip". Follow the directions on the bottle exactly – no fudging with this stuff.

On a wood deck, look for decayed wood. If the wood is soft when poked with a screwdriver, or sags when you step on it, it is probably rotten. Replace any rotten deck boards and structural members. Note the spots where the deck seems excessively worn. These areas are candidates for an extra coat of stain and sealant.

Look for visible patches of mold (usually black) and mildew (usually green). Mold and mildew are everywhere, so you are bound to have some of each on your deck somewhere. But, large patches often indicate places where cleaning with a fungicide might be called for.

This inspection is very important on composite decks that are very susceptible to mold and mildew buildup. Real wood has natural fungicides built-in, so deeply rooted mold or mildew is rare. Not so on composite decks. The material in the composite includes a "wood flour" as a primary ingredient. All of the natural fungicides have been removed from the flour during processing, so composite decks can develop truly awesome bumper crops of mold and mildew that only concentrated fungicides can remove at the roots. If you don't get the roots, it will, like the Terminator, be back.

Step 2: Preparation

Sweep debris like leaves, twigs, and loose dirt off the deck. Rinse the deck to help loosen heavy deposits of dirt and mud. Rinsing the deck before applying deck cleaner will cool the surface and help prevent the deck cleaner from drying too fast to be effective. This is especially important for composite decks that can get very hot in summer.

Wet down plants and grass around the deck and, if necessary, cover plants, grass, concrete, and other areas you want to protect from potential damage or staining. Plastic sheeting at least four mils thick (a mil is 1/1000 of an inch) is adequate to prevent incidental damage by pressure spray or chemicals.

Clear plastic works best because it doesn't allow as much heat buildup and you can better see the plants under the plastic and any water or chemical pools that may build up on top of the plastic. Keep in mind that there is absolutely no deck cleaner that plants actually like.

Step 3: Washing the Deck

Because they are outdoor horizontal surfaces, decks collect a lot of dirt, grime, mold, and mildew. A yearly washing keeps the deck surface looking nice and is a necessary step before sealing or staining. Power washing is seldom necessary and excessive pressure from the power washer may actually damage the deck. General washing with a good cleaner using a garden hose will usually suffice. If your deck is so dirty that a power washing is necessary, leave it to the professionals. They have the equipment and know-how to clean your deck without harming it. By professionals, we don't mean the guy in the pickup truck with the Sears power wash. We mean the guys in the Better Business Bureau list of accredited cleaning professionals.

There are many deck cleaning products available. They are all some variety of detergent and their job is to loosen dirt and grime so it can be rinsed away. There are three general types:

Bleach-Based Cleaners These cleaners usually contain chlorine bleach (sodium hypochlorite). Chlorine bleach is commonly used to kill mold and mildew. Bleach appears to kill and remove the visible signs of mold and mildew but actually has little effect on the mycelium or roots of the fungi. After a few weeks, mold reappears. Other products, made just for decks, are more effective.

Chlorine bleach is highly corrosive to any metal including structural connectors or fasteners (screws) holding the deck together. It may shorten the life of the pressure-treated lumber supporting the deck.

Bleach-based cleaners may leave an unnatural whitish appearance, and sometimes leave the wood a little fuzzy because it breaks down the lignin in the wood. Lignin is the fiber that holds the wood together. A fuzzy deck may require sanding after the wash.

Special care needs to be taken to protect plants and adjacent concrete. It will quickly kill plants and leave white spots on concrete.

Lye-Based Cleaners These are not do-it-yourself products. These are powerful cleaning agents based on sodium hydroxide, also known as lye or caustic soda. It is a strong grease remover and is the primary ingredient in oven cleaners. We don't think these are available in retail stores — we hope they aren't. These require training and experience to use safely. They can damage your deck and need to be neutralized after use with an acid-wash solution. These cleaners are best left to professionals with experience in their use.

Warning: First, Test All Cleaners

Test all cleaners on an inconspicuous spot on the deck before using them.

Even if you have used the cleaner before, test it again. The manufacturer may have modified the formula.

We rarely use them and only if there is serious discoloration and we intend to restain the deck. If your deck is so deteriorated that it needs this kind of treatment, hire a professional with the right equipment and protective gear.

Acid-Based Cleaners Most of these contain some form of oxalic or phosphoric acid. They are used primarily to remove tannin stains that naturally occur in redwood, cedar, and oak, and work very well as spot cleaners to remove rust stains.

On wood decks, this type of cleaner is used as a supplement to oxygenated cleaners. However, many composite deck manufacturers recommend acid-based cleaners for their products. You are generally better off following the manufacturer's recommendations. But, be careful with acid-based cleaners. They can leave the finish on a composite deck looking cloudy or "powdery". If you have a choice, use a citric acid-based cleaner in preference to one based on oxalic acid. They are generally a little milder and a whole lot more eco-friendly.

Oxygen-Based Cleaners Oxygen-based cleaners are somewhat milder but nearly as effective and more environmentally friendly than chlorinated products. When mixed with water, these create a hydrogen peroxide and soda ash cleaning base. This is effective in removing mildew stains and graying caused by ultra-violet sun exposure on the wood and in cleaning the wood of dirt and deposits. The wood usually returns to its original color after cleaning with this type of cleaner.

Whatever cleaner you use, follow the manufacturer's directions carefully. Some require the deck to be wetted before application. Most recommend that you clean on a day when it is not too hot and out of direct sunlight. Many if not most will damage surrounding plants, which need to be covered and protected. Most cleaners are concentrated and require dilution before use. The amount of time the cleaner must sit on the deck varies as does the amount of waiting time before a new sealant can be applied.

Wear eye protection and closed-toe shoes at all times when using these products.

Apply the cleaner to the deck. Most can be applied with a garden sprayer. Work a small section at a time - not more than 10' x 10' - smaller if this is your first time. Let the cleaner set for the amount of time recommended by the manufacturer — usually between 10 and 20 minutes. After it has set, scrub the deck thoroughly with a good, stiff, natural-bristle scrub brush to work the cleaner into the deck crevices and help loosen crusted on dirt, mold, and mildew.

Pay special attention to places where there is a buildup of mold and mildew or heavy discoloration.

Rinse thoroughly using lots and lots of freshwater. You want to remove all traces of the cleaner. Don't forget to rinse the underside of the deck boards and all the structures under the deck.

If the deck is still not clean, do it again.

Once the deck is cleaned to your satisfaction, allow the deck to dry for the number of days recommended by the manufacturer of the sealant you intend to use — usually two days. If it rains during the two days, restart the count on the day after the last rain.

Step 4: Staining and Protecting

Deck stain and sealant terminology is confusing. There are stains that are just stains, without any sealant properties. There are stains that are also sealants. These are both usually called "stain" or "penetrating stain" on the can. You have to read the can to find out which is which. You can, especially on a new deck, use a non-sealing stain then cover it with a sealant. But, combination stain/sealants are so good these days that there is no reason to go to all the extra work. Penetrating sealants are classified by how much pigment they contain, and their formulation.

Pigment Classes

There are five general pigment classes indicating relative opacity:

Look for the Master Painters Institute (MPI) Certification

Ensure that whatever sealant you use clearly states on the container that it is water repellent and meets Master Painters Institute (MPI) standard #33 for deck sealants. You will have to comb through all the fine print on the can to find this approval stamp but if it's not there, don't buy the sealant.

MPI Seal

A great many heavily advertised deck sealants do not meet this standard, so look for it even if the brand you are buying is well known. For a list of sealants tested and approved by MPI, click here.

Water or Oil Based?

Sealant formulations are either water or solvent (oil) based. Water-based sealants are easier to use but most professional sealants are oil-based. They work better. There are also epoxy-based sealants but these are quite expensive, difficult to work with, and have yet to prove any more effective than oil-based sealants. Oil-based sealants clean up with mineral spirits or paint thinner — so when you buy the sealant, also buy the solvent.

Applying the Sealant

You can apply most sealants with a brush and roller or garden sprayer. Most pros use a special applicator pad that makes the job go a little faster. You can get one of your very own at your local paint store.

If you spray, you will also want a brush to work the sealant into corners and crevasses and a roller to spread out any puddles. A sprayer is difficult to control, so make sure everything you don't want to spray is completely covered with 4 mil plastic sheets The sealant will happily stain your patio furniture and concrete patio just as thoroughly as any wood on your deck, so be careful. It's not as easy as the pros make it look.

Two thin coats are better than one thick coat. Do not allow the finish to puddle. To speed up the process, one person can apply the stain or sealer with a sprayer and another person can use a roller or brush to spread puddles and to work the finish into the wood, a process known as "backrolling" or "backbrushing".

When you are done, rinse off any tarps or ground cloths with the hose before spreading them out to dry. If you used any rags for cleanup of a solvent-based sealant, spread them out to dry completely. Heat generated from evaporating finish on a pile of rags can start a fire. If you are using water-based stain, clean equipment with soapy water. If you are using oil-based products, clean with paint thinner or mineral spirits. (Learn the proper way to clean a paint brush.)

Protecting a Composite Deck

The days when composite decks were sold as "no maintenance" are gone. Too many composite suppliers got sued too many times. Now, the most manufacturers will say about their composite decks is that they are "low maintenance" or "lower maintenance" than wood decks. (We are not at all convinced that even that limited claim is wholly true. See The Deck Handbook, Part 4: Composite Decks.

Most composite manufacturers recommend that their products be cleaned and a protective coating applied once each year — exactly the same routine maintenance recommended for wood decks. The coatings are similar to sealants for wood decks but with a slightly different formulation. Look on the website of your composite supplier to find out which sealant it recommends.

Composite coatings, like wood sealants, retard mold and mildew with some sort of fungicide and provide UV protection that slows fading. While no composite manufacturer yet admits that composites need to be stained every once in a while, the paint companies, like Sherwin-Williams, have produced some pretty convincing evidence that many composite decks will fade to gray in just a few years. Restoring the original finish requires re-coloring.

Composite deck stain formulations are not much different from wood stains, and frankly, we feel a good wood stain and sealant would work just as well. However, for the small difference in price, select a stain that is specifically formulated for composite decking.

Most manufacturers suggest the stain be applied to a clean deck every two years. We think you can stretch that to every 3 to 5 years depending on where you live and if there are any trees growing over your deck.

Rev. 05/20/20