Why is moisture a problem while coating?
August 21, 2020
Successfully coating or re-coating a deck has a lot to do with the moisture in your timber. Knowing the moisture content before you start will generally minimise the most common reasons for failed coatings. If you are unsure, or live in a wet climate, we recommend getting a moisture meter. Bunnings sell them from $25, which is cheap compared to getting it wrong!
Moisture is not necessarily your enemy, though.
Timber is a natural product. Treat it that way.
That’s why we love it, right? It’s a natural building material which has been in service since humans started to construct the most rudimentary of shelters.
However, being natural, it needs maintenance, protection and some understanding.
A tree uses the xylem (little tubes) to transport moisture using capillary action. Therefor timber has a natural moisture content. The moisture content may be as low as 40% but can be as high as 180%. Once we cut the timber, and mill it, some of this moisture naturally dissipates over time. Freshly cut timber is called ‘unseasoned’. That is, it has not been exposed to the elements or a kiln to dry out to a moisture content matched to its intended environment (this could be 12% for indoor flooring in Victoria and around 16% for outdoor decking in June in Victoria). It is advisable to check the Australian Standards for timber moisture for your use and your location. For particular species research, there is a great resource HERE.
Unseasoned timber is cheaper to purchase. However you need to be very aware of the possible movement you can expect due to moisture loss and detail your design accordingly. Unseasoned timber is well known for checking, shrinkage and warping.
However, even seasoned timber will absorb moisture from the environment, and be affected by weather and traffic. It’s generally recommended to protect the natural surface with a coating. On exterior decks, the preferred coating is a decking oil so you can preserve the beauty of the natural timber aesthetic (and cope with the changes in weather).
Penetrating Oil is patchy, sticky and not drying? You have a moisture issue.
Significant moisture inside timber is a problem for a coating. And the old adage that oil and water don’t mix is absolutely correct. Moisture, either in the timber or on the surface, makes it difficult for a penetrating oil to go beyond the surface. If oil does not penetrate, it can leave a film-like coating on the surface that doesn’t bind well with the fibres, (ie. expect flaking and patchiness), and it has a hard job protecting the timber because effectively there is still water between the oil and the wood fibre. If you continually re-coat on top of this situation, the result is not going to be pretty! It’s like a face cream applied to wet skin, which behaves just like an oil slick. Timber behaves similarly.
Traditional oil coating products require a low moisture content in the timber for the oil to penetrate better. The Australian Standard is a 9-14% moisture content. However, it takes weeks for unseasoned timber to dry to this level. Timber is also naturally hygroscopic, which means its moisture levels are affected by the air around it and the percentage will continually rise and fall depending on climate conditions. In a highly humid or wet environment, it may never reach an ideal percentage.
Equisol’s stabilising oil has been developed with special moisture penetrants to allow the oil through the water barrier where it can be absorbed. It allows the inherent moisture in the timber to evaporate over time, and lets humidity back in – letting the timber breathe!
Without this technology, other oils cannot penetrate the water barrier, and don’t dry, eventually becoming sticky and patchy. Some can take weeks to dry, and leave oily residues. Equisol is dry enough to walk on within 48 hours.
Equisol can handle higher moisture levels.
We have successfully applied Equisol to decks with moisture content as high as 24%. These were fairly extreme conditions and the circumstances dictated the risk.
Equisol’s superior moisture control comes with some added benefits too.
FASTER DRYING. This minimises things that can go wrong while you are waiting for the timber to dry.
ABSORPTION. As the oil gets absorbed it will not leave wet edge marks. Dirt or leaves will also not stick during application process.
NO PATCHES. Shady areas will suffer from patchiness. Non-uniform moisture content across a deck (for instance shady zones) are often troublesome for surface coatings.
Plus, Equisol can work in a wider moisture band, so you can coat on days that are hot, sunny, humid or cold. Depending on where you live, that gives you more flexibility and less concern about the weather turning inclement half way through your coating or drying time.