Last Updated on May 17, 2023 by Ernest Godia
Wood in its natural state is susceptible to rot, mold, insect, and water damage. Pressure treatment is one of the methods used to preserve wood and make it more resilient against weather elements.
Pressure treatment involves forcing fire retardants and preservatives into the wood. These preservatives protect the treated wood from insect attack and weather damage. As a result, pressure-treated timber can last much longer than untreated wood in any environment.
But does pressure-treated wood need to be sealed, or is the treatment enough to protect it against elements?
This article looks at sealing pressure-treated wood, whether it needs sealing, and how to seal it. Learn how to seal pressure-treated wood on your deck, porch, or fence.
Does Pressure-Treated Wood Need To Be Sealed?
Pressure-treated lumber contains chemical preservatives in its wood grains, making it resistant to insect damage and rot. However, the longevity of pressure-treated wood depends on its treatment level and intended use.
Pressure-treated wood is protected, but there is no harm in sealing pressure-treated wood, especially on outdoor wood. The sealant provides additional protection against harsh weather conditions, moisture, and seasonal changes.
What is Pressure Treated Wood
Pressure-treated wood is any lumber that has undergone pressure treatment to make it more durable and resistant to mold, insect infestation, and water damage. Such wood is ideal for the outdoors, like on decks or fences, as it is stable and resistant to the elements.
There are various types of pressure treatment, and the level of treatment dictates how and where you can use the wood. In any case, the quality of the treatment the wood receives depends on the amount of the chemical injected into it.
Some chemicals used in pressure treatment also act as fire retardants, making pressure-treated wood resistant to fire.
When working with pressure-treated wood on your projects, confirm if the lumber is dry, as the chemical compounds used in the treatment can cause skin irritation and other health-related issues.
The wood comes with tags or stamps indicating whether it was air-dried after treatment (ADAT) or kiln-dried after treatment (KDAT).
Where To Use Pressure-Treated Wood
There are three locations where you can use pressure-treated lumber, they are:
- Above ground: This type of pressure-treated lumber is ideal for above-ground projects like fences and construction. It contains preservatives like Copper azole type C (CA-C) which repels water and others, making it less prone to insect damage and rot.
- Marine lumber: This type is perfect for areas with constant and direct exposure to water. It is the ideal pressure-treated wood for construction around the seaside and docks.
- Ground contact: As the name suggests, this type of pressure-treated wood contains preservatives making it last approximately 40 years in the ground. The preservatives make the wood resistant to water damage and rot.
How is Pressure Treated Wood Made?
Wood is stacked, placed into a cylinder, and subjected to high-pressure treatments. Pressure-treated lumber undergoes the following high-pressure treatment cycle:
- Once the timber is in the treatment vessel, an initial vacuum evacuates the air out of the timber cells.
- The cylinder vessel is then flooded with Tanalith E, a chemical wood preservative, and other additives added, like color and water repellents.
- The preservative treatment utilizes hydraulic pressure to enable chemicals to penetrate the wood grains.
- The cylinder is vacuumed again to extract any excess preservative solution and pump it out for later use.
- Then the vessel allows in a standard atmosphere, and the low pressure of the timber cells draws in any preservative on the surface into the wood.
- Finally, the cylinder is opened, and the treated timber is placed in a designated drying area.
Characteristics of Pressure-Treated Wood
- The chemical preservatives in pressure-treated wood make it more resistant to the elements, rot, and insect damage.
- Pressure-treated wood is available in grades, and the higher the grade, the greater the surface area of wood covered with the preservative and the fewer the defects.
- Newly pressure-treated wood is green in color, but with time, it dries golden honey. As it ages, it develops a silvery patina.
How To Seal Pressure-Treated Wood
Previously manufacturers advised against sealing pressure-treated wood as it was notoriously challenging.
Today’s pressure-treated wood is relatively easy to seal if done correctly. Here is a step-by-step guide on sealing pressure-treated wood like a pro:
What you’ll need
- Bleach
- Commercial cleaning products
- Natural bristle brush
- Sealant
- Scrub pad
- Protective gear
STEP 1: Remove stains, mold, or mildew and any old finish on the wood
Inspect the wood for mold, stains, and mildew. The chemicals used in pressure treatment increase the wood’s ability to retain water, making it prone to mold.
If the mold growth on your wood is widespread, make a homemade solution of water and bleach in the ratio of one part bleach and three parts water. Clean the wood and remove the fungus.
When working on large surfaces like decks of porches, pressure washing is a better alternative. Maintain a pressure of 1500-2500 PSI to avoid damaging the wood.
STEP 2: Prep the wood surface and let it dry
The best way to clean pressure-treated wood is with a scrub pad and a professional cleaning solution. Letting the wood dry for approximately 48-72 hours after a thorough scrub and rinse would be best. Use commercial products labeled as restorers, cleaners, or brighteners.
The wood needs to be completely dry for easy application of the sealant. However, you can use Thompson’s Water Seal Advanced Wood Protector on moist lumber to clean and seal your pressure-treated wood the same day.
STEP 3: Select your preferred sealant
At this point, you have your mind on a particular finish. Water and oil-based sealants are available in transparent, solid, or semi-transparent forms. The application requires a high-quality natural brush, foam, roller, or paint pad. You can also decide to use a spray-on sealer to save time.
If you want to preserve the natural beauty of the wood, work with a clear or semi-transparent oil-based sealer.
To change the color of the wood and blend with your décor, use a solid or combine a stain and sealant.
We highly recommend the 100% acrylic finish formula as it’s extra durable, easy to apply, and enhances the wood’s natural appearance.
STEP 4: Apply the finish
Apply the transparent or semi-transparent sealers with a sprayer or buy a spray-on. Spray the wood with even thin layers of the sealer while retaining the natural appearance of the wood.
When using a brush, the result is outstanding if you follow the wood grain. Natural brush strokes are uniform as the bristles fill the open pores and grains evenly.
Keep a wet edge with minimal overlapping, and wipe off any drip and puddles forming as you apply the sealer. Work on a maximum of four boards at a time.
STEP 5: Let it dry
Pressure-treated wood takes about three days to dry completely. The drying time depends on whether you are using oil or water-based sealant. You can use a tester on the wood to check the moisture level of the lumber.
Read the weather forecast prior, as there needs to be no rain in the next three days till the wood is dry. At this point, you can add a second layer if need be.
Methods Of Sealing Pressure-Treated Wood
Below are ways to seal your pressure-sealed wood:
Method 1: Applying a preservative
Preservatives are added to pressure-treated wood to make it resistant to moisture, weather changes, and insect damage. The type and amount of preservative the wood absorbs influences its durability and usage.
Some of the common preservatives are:
- Creosote: Protects the wood against rot. It isn’t allowed for residential use.
- PCP: It works as a pesticide and disinfectant.
- Borate: It’s a good preservative, but avoid it in areas with high water exposure as borate can leech out of the wood.
- Copper azoles: These include copper azole type B and type C, which combine copper and azoles, act as a preservative, and protect the wood from decay.
Method 2: Using waterproof paint
Applying a layer of paint on the wood is an effective way to offer extra protection. Oil-based paint, in particular, is resistant to heat, moisture, and UV light, reducing the amount of damage to the PT wood.
Method 3: Adding a protective wood sealer
The best protective coats for your PT wood are polyurethane and epoxy. Both products provide additional resistance to moisture and decay. Read our article on epoxy VS polyurethane to determine what product you prefer.
Method 4: Using a protective cover or tarp
You can cover the surfaces with a tarp or plastic sheeting. Tarp is a large sheet made with waterproof, strong, and flexible material. The tarp is often a canvas or polyester coated with polyethylene or polyurethane.
Method 5: Applying a stain-sealant combo
Thompson’s Water Seal Advanced Wood Protector is a good example of a stain sealant combo. Such products color the wood and protect the surface in one go.
This article sheds light on how to seal pressure-treated wood. Coating the lumber with a sealant gives the wood extra protection and increases its longevity. The sealer reduces damage from water, insects, fungi, and sunlight.
Uses Of Pressure-Treated Wood
Here are some of the instances you can use pressure-treated wood:
1. Residential use
PT wood contains preservatives that enhance its durability and resistance to insects, water, and mold damage. These qualities are excellent for decks, docks, fences, and porch construction.
You can use pressure-treated wood in your interior spaces in heavy traffic areas or areas prone to wear and tear.
Notice that the treatment levels of PT wood used on cabinets or furniture is lower than that of outdoor projects.
2. Heavy Construction
Marine lumber contains chemicals ideal for heavy construction like on docks, brides, guardrails, or indoor pools.
Use the ground contact pressure-treated wood as posts, beams, or in projects with ground contact or where lumber touches concrete. Keep the above-ground PT lumber above ground to increase its durability. Pressure-treated wood is the best choice for your outdoor projects.
What is the Best Sealer For Pressure Treated Wood?
The top choice of sealant increases PT lumber’s durability and preserves it from moisture, heat, insect, and UV damage.
There are numerous choices of sealers available in the market, and to pick the right one, you’ll need to consider the following:
- Reliability: The sealer should protect the PT wood from moisture, heat, and damage from the elements.
- Cost: Choose the best product within your price range to avoid overstretching your pockets.
- Appearance: The type of sealant you’ll pick determines the general appearance of the PT lumber.
- Durability: Select a sealer that protects the pressure-treated wood prolonging its life.
How Long Do You Have To Wait Before Sealing Pressure-Treated Wood?
Applying a sealant on wet PT wood is pointless, as it peels off quickly. You’ll need to give air-dried after-treatment wood a few weeks to dry completely. You can use a moisture meter to determine the moisture content in the wood.
If you don’t own a moisture meter, you can still tell if the wood is dry. Sprinkle some water over the PT wood; if it forms beads on the surface, don’t stain the wood yet. If the wood absorbs the water in less than ten minutes, it is dry enough for staining.
Manufacturers recommend waiting a week to two months before sealing kiln-dried after-treatment lumber. The waiting time depends on the moisture content of the wood, weather forecast, type of sealer, and level of treatment.
Apply two coats of 100% acrylic latex paint for the best results.
When you need advice on sealing your pressure-treated wood, consult an expert at your local hardware store.
Pro Tips for Sealing Pressure-Treated Wood
Here are some tricks to use when sealing pressure-treated wood:
- Wear protective gear, as exposure to the chemicals can cause skin irritations and other health issues.
- Consider using a hybrid sealant as it penetrates the wood grains, making it durable and easy to clean with soap and water.
- If you want to preserve the natural wood appearance, use a transparent or semi-transparent sealant. Use a solid color sealer if you intend to change the entire look of the wood.
- An oil-based sealer for outdoor projects offers better protection against moisture, heat, and weather elements than water-based sealants.
- Ensure the wood is completely dry before sealing it so the coat can adhere to the wood surface.
- Monitor the weather forecast, as the sealant needs no rain for at least three days to dry completely.
- When painting vertical pressure-treated wood surfaces like fences and rails, start from the top and work downwards. For flat surfaces, seal three boards at a time and avoid overlapping.
How To Maintain Sealed Pressure-Treated Wood
The best way to maintain sealed pressure-treated lumber is by keeping the surfaces clean and resealing them every 1-2 years.
You don’t have to use PT hardwoods only for your products, as softwoods like pine and low-cost hardwoods like maple are equally attractive, cost-effective, and easy to maintain. In addition, after pressure treatment, you can make the wood more durable by sealing it.
Pros and Cons Of Pressure-Treated Wood
Here are the advantages and drawbacks of using pressure-treated wood:
Pros
- Pressure-treated wood is remarkably affordable than other types of wood like cedar or redwood.
- It contains preservatives making it resistant to rot and insect damage.
- Pressure-treated wood is durable as it is less susceptible to wear and tear, abrasions, dents, and scratches.
- It accepts stains and paints, making splashing color and matching your style effortless.
- In case of destruction or damage, pressure-treated wood is easy to repair.
Cons
- Some of the chemicals used in the pressure treatment contain toxins, making the wood less suitable for residential use.
- PT wood is prone to getting splinters over time or with seasonal changes.
- Due to checking, fading, and splintering, pressure-treated wood is high maintenance.
- With exposure to the elements, PT wood tends to fade.
Sealing Pressure-Treated Wood: FAQs
Here are answers to commonly researched questions on sealing pressure-treated wood:
Does pressure-treated wood need waterproofing?
Yes, you should waterproof your pressure-treated wood to prolong its service life. Pressure-treated lumber contains chemical preservatives protecting it from mold, decay, and insect damage. Waterproofing the wood protects it against moisture and water damage.
How long will pressure-treated wood last without sealing?
The durability of pressure-treated wood depends on exposure to the elements, maintenance, type of wood, and its application. All things considered, pressure-treated wood should take at least 40 years without signs of rot or decay. For surfaces like decks and flooring, give it approximately 10 years.
Does pressure-treated wood rot?
PT wood is still wood and natural wood is prone to decay. Pressure-treated wood will last longer than untreated wood. However, it can rot. Pressure-treated wood can form cracks, dents, or bends and allow water to sip into the wood. The water introduces oxygen and microorganisms resulting in wood rot.
How durable is pressure-treated lumber?
Woods treated with creosote can last over 50 years, while those preserved with copper naphthenate or pentachlorophenol can last at least 60 years.
What happens if you don’t seal pressure-treated wood?
Pressure-treated lumber is porous; if you don’t seal it, the surface is subjected to UV light, snow, rain, and dew. With time moisture penetrates the wood grain causing the wood to swell, and during warm weather, the wood dries and shrinks.
The fluctuating pattern of swelling and shrinking damages the wood, causing splinters and checks.
Does Pressure-Treated Wood Need To Be Sealed: Summary
Pressure-treated wood contains chemicals that make the wood durable and less prone to water, rot, and insect damage. For extra protection, it is best that you seal the wood. Sealing pressure-treated wood does more than add a splash of color to the wood outdoors, making it more vibrant.
Is your outdoor pressure-treated wood taking on a silver-gray appearance? Do you want to extend the service life of your pressure-treated wood? Do you intend to add more appeal to the PT lumber on your deck or fence? If so, seal It!