Linseed Oil: Breaking the Mold


Wood is incredible. It’s strong and easy to cut and use right away. It’s pretty and light and flexible. It smells good. It’s an ancient tried and tested building material that has been used longer than people have had words to name it. The problem? It rots. To bacteria, mold, and fungi (and some rodents), wooden 2×4’s are like huge Snickers bars. Most wood species, thankfully, realized it was not ideal to be thought of as a large assortment of food by microscopic organisms and started developing natural defenses in the form of tannins, oils and resins which make it taste awful (awfully poisonous) to tiny bacteria. Evolutionary success at its finest!

However, the passive mold resistance in some wood species had the unintended side effect of making it more desirable to be grown and harvested as a building material (sorry, pine trees). Cedar is a good example of a tree that bacteria don’t like to eat. It is so full of these microbe-fighting tannins that it actually corrodes most metals, like iron, zinc, and aluminum.

I’m not kidding. You can waste a lot of money on nails and find only a few years later that they turned into rust in cedar siding. That, and the spot around the nails looks like a big black bleeding spot. Pretty. This is why we chose stainless steel fasteners for the cedar exterior. They have so little iron content that they don’t even stick to the magnetized tips of our screwdrivers. That’s the reason they’re some of the only fittings allowed in marine grade construction, the next being ceramic coated or otherwise specially treated screws or nails.

Stainless steel fittings may be a bit expensive, but for a long lasting exterior, they’re worth it. They never corrode, they’re strong, and the cedar stays where you want it year after year, decade after decade, until it grays out and starts looking like a traditional silvery cedar lined barn or sea-side shack with cedar shakes. In these cases, even unfinished cedar stands up to the constant fungal and bacterial onslaught… but in our humid, bio-diverse climate, life always finds a way. This is why we decided to coat our boards with a natural finish called boiled linseed oil.

Linseed oil is a finish you’re probably familiar with, or at least heard of somewhere. It’s an old finish used by furniture makers, Dutch carpenters, and painters. It’s the rich golden oil expelled from flax seeds that has been boiled so as to reduce the drying time and make it thicker. It’s safe to use, non-toxic, doesn’t off-gas or peel like modern polyurethane coatings, and the best part: it’s fairly affordable at $20 a gallon, compared to stains or paints. Painted onto an exterior board, it has been known to preserve the wood’s natural grain pattern and color, and has prevented warping or splitting for 20-some years on a single coat. If you ever need to reapply it, as it does completely soak into the wood and dry out over time, the process is easy and doesn’t require a painter.


We originally heard about linseed oil from a local builder who used it on his own house. He has a section of pine boards on his stucco exterior that he has kept regularly oiled since its construction. Great!

Here’s the fun part: his house has been around about a year or two now and he’s already run into some problems using linseed oil. For one, he discovered a local mold growing near the base of his boards where splash-back occurs during rainstorms. It’s not something he can really sand off since it’s inside the wood. Not only that,but as he was inspecting the rest of his siding he kept finding all different kinds of colored molds and fungus living in it! Yikes!

He spoke with me a while after this happened. We theorized it was because he had used pine on the exterior, and while pine does have some protective tannins, it doesn’t have nearly as many as cedar. This is why cedar has been used for centuries as an exterior wood. But, even with all of cedar’s tannins, we’ve noticed some light spotting around the lower boards on our house. Luckily it isn’t a dangerous mold. It just lives in the wood and creates a minor cosmetic headache. Building in such a bio-diverse area has made the mold and fungus population so strong that even cedar does get some spotting.

Since the builder’s discovery, he’s stained his walls with actual, heavy-duty clear-coat in order to keep his natural, unpainted look. While this worked for him, this isn’t something we would like to do because we’re trying to avoid toxic chemicals.


Currently we’re considering painting our house with an oil-based, non-toxic paint in order to seal our house and protect it. But then this will get rid of the natural look we’ve liked so much in unfinished cedar. At the moment, we’ve settled on re-oiling the exterior boards that have dried up since we last worked on the house. This should create a temporary barrier until we can come up with a better solution.

The saga continues!


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s