A few weeks back, here on our blog, we talked about the proper steps for cleaning and preparing a deck for restaining. We briefly touched on the importance of soaking a deck with water before using chemicals to clean the wood surface. We didn’t delve into many of the finer details, though. Today, we’ll discuss more about the chemical action and the process of proper cleaning.
Wood decks need to be cleaned from time to time to remove dirt, debris, and organic materials that can stain or discolor the surface. Once things build up excessively, they’ll actually retain moisture longer than if they’re properly cleaned, so cleaning should happen once in a while. Common culprits include algae, mildew, mold, and moss, which thrive in moist environments. These organisms not only discolor the wood but can also make the deck slippery and unsafe. As well, dirt, pollen, and other airborne particles often accumulate which can create a grimy appearance and potentially harbor microorganisms that also further contribute to staining. So this staining cycle can be a bit of a vicious cycle. Algae and mildew particularly thrive in shaded areas where sunlight is limited, and moisture tends to linger. Wood’s porous nature allows it to absorb water, creating an environment for these organisms to grow. Wood is a hard and relatively heavy material, so we don’t think of it on a microscopic level, but when you look very closely at it wood is made up of a cellular structure, not that different to a very tiny sponge. This porous latticework structure of wood fibers can take on a significant amount of moisture. Other materials in nature like stone or earthen materials can also be porous and take one moisture to a degree, but wood takes on a particularly high amount of moisture and hydration. Over time, this growth can lead to green or black stains that are difficult to remove and can damage the wood if left untreated. Similarly, fallen leaves and organic debris can trap moisture against the deck wood, promoting fungal growth and leading to further discoloration and decay. When wood absorbs moisture from rain, humidity, or even dew, it provides the damp conditions and environment needed for mold, mildew, and algae to thrive. The continuous wet-dry cycle can also lead to the wood expanding and contracting, causing cracks and splits where these organisms can take hold more easily which is why sealing (after the first few months (and later resealing) matters. Regular cleaning helps to prevent these issues by removing the organic materials and dirt that support the growth of these organisms, keeping the deck looking clean which extends its lifespan. Sometimes though, regular cleaning isn’t enough and chemical cleaning is required .
Since wood, as we explained above, is a porous material, composed primarily of cellulose fibers, these fibers can absorb liquids readily, especially when the wood is dry. When a chemical cleaner is applied to a dry wood surface, it can penetrate deeply into the wood structure due to capillary action and the concentration gradient between the surface and the interior of the wood.
The depth of penetration depends on several factors:
- Wood species and its specific gravity
- Grain orientation (radial, tangential, or longitudinal)
- Moisture content of the wood
- Viscosity and surface tension of the cleaning solution
- Exposure time
Wood species
There are multiple different types of wood species or families that trees can be grouped into, but to generalize a bit, most trees can be grouped into two distinct classes of coniferous and deciduous trees. Coniferous trees are those with pine cones and or needles, in most cases. These trees generally have wood that is relatively soft and by comparison, deciduous trees generally have harder wood with tighter grain. Deciduous trees are generally the ones that lose their leaves in the fall and winter seasons.
Grain Orientation
The difference in wood grain, to just make it very simple, can generally be viewed as follows: regular boards, when stained or chemically treated on the longitudinal face, don’t absorb chemicals or liquids as readily as they do at the end-grain. This applies to all sorts of different circumstances and conditions such as exposed end-grain on a cutting board or countertop surface, oil and finishing materials will absorb much better and faster.
Moisture Content of the Wood
Earlier in this article, above, we mentioned that you can’t stain and seal of wood deck as soon as you install it, depending on the type of wood and the level of treatment and chemical exposure. For most typical wood treatment applications today, wood deck materials require about 3 months of time to season before they can really be stained and sealed. Stains and sealers are two different things, in most cases, and a stain basically just provides a color, but it also can help to protect the wood from UV exposure. Ultraviolet rays and sunlight and break down wood materials, to a degree. So stains can help, but they don’t actually do the same thing as a sealer. A sealer can protect the wood surface wood material below the surface, to a degree, from exposure to moisture. Wood sealers essentially can help a deck keep water from seeping deep into the wood fiber. There are differences though, depending on the type of wood used to build the deck. Treated Woods, depending on the type of chemical used in the treatment, have a high degree of moisture, at the start. Other materials, though, like exotic hardwoods woods, will generally have a lower amount of moisture content and therefore will allow chemical treatments to enter more deeply into the wood at an earlier point after installation, with significant variables such as fiber density.
Exposure time
Different chemical cleaners have different amounts of exposure times required, one of the leading factors is the ambient temperature and or amount of dirt and grime present in the wood in the amount of deterioration or age of the wood decking.
When chemical cleaners penetrate too deeply into the wood, they can cause several issues, such as the following:
- Difficulty in complete removal during rinsing
- Potential chemical reactions with wood components (lignin, hemicellulose)
- pH changes within the wood structure
- Accelerated degradation of cellulose fibers
These issues can lead to long-term problems such as: discoloration, reduced effectiveness of subsequent stains or sealers, decreased dimensional stability, and increased susceptibility to fungal decay.
Pre-wetting the deck helps mitigate these risks. When wood fibers are saturated with water, they cannot readily absorb additional liquids. This phenomenon is due to the principle of osmosis and the reduction of the concentration gradient. The water molecules occupy the interstitial spaces within the wood structure, preventing the chemical cleaner from penetrating as deeply.
The benefits of pre-wetting include:
- Localized cleaning action (primarily on the surface)
- Easier rinsing and removal of chemicals
- Reduced risk of wood fiber damage
- More predictable and uniform cleaning results
It’s important to note that while pre-wetting is beneficial, over-saturation should be avoided. The goal is to achieve fiber saturation point (FSP) near the surface, not to soak the entire board. FSP is the point at which the cell walls are saturated, but no free water exists in the cell lumina. This state allows for effective surface cleaning while minimizing the risk of trapping moisture within the wood, which could lead to decay if not properly dried before finishing.
In this coming week’s blog article, we will also look at the different types of options related to chemical cleaning of a wood deck before resealing.
Use a contractor who understands and cares about doing things right. Always, feel free to reach out to us here at Dupont Decks and Patios. We are happy to help with almost all steps of the deck building and design process. Let us know about your ideas and talk to us if you have questions about possibilities . You can call us at (202) 774-9128. You can find us online at https://dupontdeckspatiosdc.com and you can email us there as well at https://dupontdeckspatiosdc.com/contact-us.
