What is Impregnated or pressure-treated wood?

Impregnation and pressure treated wood are the same. The process is identical and is usually performed outdoor for rot protection. The two terms used describe the same wood treatment:

Impregnation is done using impregnating agents; a well-known brand of an impregnating agent is ‘Tanalith E.’ This brand has been known since the 1940s. Pressure-treating is the process in which a product like ‘Tanalith E’ is used.

When impregnating wood, the wood is treated with a combination of chemicals, with copper as the main component.

The metal copper protects the wood excellently against wood rot and insects. The other added substances preserve the wood against other forms of attack, which the copper does not influence on.

Impregnated wood often has a characteristic green glow, and this is due to the copper that is now in the wood.

pressure treated wood

Two common ways to impregnate wood

by dipping or by applying vacuum pressure-treated wood. In both cases, an impregnating agent is used to preserve the wood, and the way is just different.

Impregnation of wood by dipping

The dipping technique is a popular technique for preserving garden wood. This technique is used for black impregnation.

Wood is immersed in an impregnating agent in a large bath for a few tens of minutes. This allows the wood to absorb as much impregnating agent as possible. Sometimes the wood is so fresh, and therefore still a bit damp that it absorbs the impregnating agent less well.

The quality of the dipping technique, therefore, depends on the moisture content in the wood. It also makes it difficult to get an even color.

Nevertheless, this method gives the wood a beautiful black color and leaves the wood structure visible. When planks are placed undercover, for example, under a roof, they will discolor to a minimum.

When impregnating through dipping, the wood is immersed in a bath in impregnating agent. The wood is then given a few minutes to absorb the impregnating agent.

This treatment method is very popular but produces less durable wood than wood impregnated under vacuum pressure.

Impregnation of wood by vacuum pressure treatment

Wood is placed in a large boiler, which is then vacuumed. The tank is then filled with an impregnating agent, ‘pressed’ into the wood by the pressure; that is why it’s said as “pressure-treated” wood.

This ensures that the impregnating agent penetrates deep into the wood so that it is even better protected.

A plank impregnated under high machine pressure lasts about 2x longer than a dip-impregnated garden plank.

Impregnation method              MaintenanceNeed paintingPainting possible
Hand impregnatedSemi-annual inspection and maintenance paint layerYes, immediately after impregnationYes
Dip impregnatedAnnual inspection and maintenance paint layerYes, within 1 – 1.5 yearsYes
Pressure impregnatedClean the bottom of the pressure-impregnated garden shed, occasionally with a soft hand brushNeverYes

Pressure-treated wood chemical

The main component is copper, to which other chemicals have been added. Copper is excellent protection against wood rot and insects.

The other chemicals (biocides) protect against different types of wood rot that copper has no influence on, such as brown rot fungus.

Pressure treated wood making wood rot-resistant

The product exactly does what it promises, and it protects the wood excellently against rot.

Internally, the protection period is about 60 years; externally, it is said about 30 years. It’s pretty good stuff!

The Limitations of Pressure Treatment

In the process explained above, the wood undergoes the following changes:

  • The color is light green in the beginning.
  • It discolors to light brown due to weather influences.
  • The wood eventually acquires a natural silver-grey color.

This color change is by no means an indication that the preservative protection is diminishing. You can then provide this wood with decorative finishing layers in the color of your choice.

If the wood has been sawn, notched, etc., similar treatment is required to protect the wood.

Defects found in pressure treated wood

Sometimes, a customer finds flaws in the process, as in the review above, without understanding much of it. Some of the following defects were found:

Formation of salts

With pressure-treated wood, salts sometimes appear on the wood surface; this resin turns yellow/green due to impregnation. These spots are gradually disappearing.

Suppose there are green spots on the wood; there’s nothing you can do about this: Just keep in mind, as this is quite normal.

Mold and Mildew

Wood impregnated by boiler pressure induction becomes very damp during treatment. As a result, the wood can be susceptible to mildew or mold, especially during warmer seasons.

These external imperfections on the product disappear on their own but are also easy to remove manually.

Since wood can expand and contract because it is a natural product, the dimensions stated in the catalog and product pages on the internet may show slight deviations.

You may also see a small crack in the wood, which is quite normal.

How to care for Mildew and the formation of salt crystals

All mold, stains, salts, etc., can be washed off or left on; eventually, they disappear. It is a standard feature of wood that has been pressure-treated.

“Don’t scrub off the splatter!” it will remove the protection. You can safely remove resin drops with a sharp knife, but do not scrub the surface.

As we now know, these are not spattered, but they are intrinsic properties of wood and are a normal result of the impregnation process, in which the copper component reacts with the moisture and sap in the wood itself.

Slight fading after the pressure-treated process

Sunlight influences wood and little can be done about it; During storage, some areas may be covered due to packaging, positioning, etc.

Other areas may be exposed to light. Light triggers a reaction in the wood, turning it brown first and then silvery. Again: nothing you can do about it, and you better take it to consider.

You can paint or stain pressure-treated wood in the color of your choice. Impregnation is, of course, a treatment against wood rot and NOT a decorative finish.

Paint or stain pressure-treated wood

Staining impregnated wood seems a bit doubly like. Wood that has been impregnated has already been treated. Impregnation ensures that the wood lasts longer.

Why would you still stain pressure-treated wood? Due to the impregnation process, the wood is protected against influences, so additional treatment is unnecessary for principle.

Thanks to the impregnation, this wood will last for 10 to 15 years. Please note that this wood is not protected against every external factor.

Discoloration may occur under the influence of weather and light. To prevent this, you can treat the wood with special oils.

Do you have to stain pressure-treated wood?

We mentioned above that impregnation positively affects the durability of certain types of wood that could use extra protection.

However, this protective layer does not mean that the wood is protected against any external factor.

For example, extreme weather conditions, from rain to bright sun, can still reduce the quality of the wood, and at some point, the wood will start to gray. To avoid such problems, choose a high-quality stain.

Stain or oil pressure-treated wood?

If you mainly expect that the wood will suffer from the sun’s UV radiation, then treating it with only oil is sufficient.

Are you afraid that the wood will gray due to the rain? Then make sure that you protect the wood well with stain or oil that you check annually for defects.

If you maintain it properly, your wood will not age. If it does become grey, you can easily get the wood’s original color back with a degreaser before you protect it again with a new layer of oil or stain.

How can you stain pressure-treated wood?

Fortunately, staining pressure-treated wood is not too difficult a task. Start preparing the substrate. Remove green mold if present.

A solution of water and soda is sufficient, but there are also special products available for this.

Then clean the entire surface and make sure that you degrease the surface. If necessary, you can use a high-pressure cleaner to clean the wood, for example, to (re)stain your fence.

After the wood surface has been thoroughly cleaned, it is recommended to let it dry and then sand it well. After sanding, it’s a good idea to dust the wood to remove all the dust.

If there is soil around the fence you do not want to pollute, it is recommended to cover the ground beforehand. Now you can get started with the stain. It is up to you which stain you choose.

For example, you can stain pressure-treated wood black, but staining wood white is also an option. You choose what to do with this surface.

If you are going to paint large surfaces, it is best to use a roller or block brush. Use a flat brush for smaller surfaces.

Advantages of pressure-treated wood

Impregnation of wood is especially useful for wood species that are not sustainable in themselves. This treatment method is often applied to wood species such as Larch Douglas, Spruce, and Pine.

Impregnation makes these types of wood suitable for outdoor use, while they cannot be used untreated.

This treatment also has many advantages:

  • It protects the wood from damage
  • Pickling is not necessary
  • It can be painted in the desired color
  • Water repellent

Harmless to the environment.

The impregnating agents used must comply with strict rules. For example, these agents have been tested for their influence on people, animals, and the environment.

That makes professionally impregnated wood safe to use. Moreover, this wood comes from sustainably managed forests and is nearby, limiting transport pollution.

There is also softwood on the market that has been treated without chemical agents. You will find this thermally modified wood; read this blog with more information about it.

What types of wood are used for pressure treatment?

Spruce and pine are the most commonly used softwoods for pressure treatment. Wood is widely available because it comes from sustainably managed forests.

For every tree cut down, a new one is replanted so that there is no total deforestation.

Untreated, these fast-growing woods are not suitable for outdoor use and are not sustainable. That is why chemical preservation (impregnation) triples the lifespan of the wood (from five to fifteen years).

Colored pressure-treated wood

pressured-treated wood can be found on the market in different colors. Wood in the colors brown and black are often slightly less durable than green impregnated timber because the dipping technique is used for brown or black impregnation.

In addition, dipping wood results in a slightly less even color. Boiler pressure impregnated wood is more green.

In all cases, the color of the wood will eventually gray due to various weather conditions unless you treat the wood with stain or paint. The graying of the wood does not mean that the protection of the treatment decreases.

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