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Why Does Wood Turn Grey?

Wood grays from both bacteria growth on its surface (mold and mildew) and UV light damage to its exposed surface cell structure.

Mold & Mildew

Wood, coming from being a tree, is an organic material. When a tree has been cut into boards they are a food source for mold, mildew, and, ultimately, wood-rot fungus. While wood rot fungus thrives in dark, damp conditions (particularly below ground), mold and mildew can thrive in drier though still periodically wet, conditions. Like watering a patch of dirt which will then quickly grow plants, periodic wetting of wood, along with access to air, grows bacteria on the surface. When wet this layer of bacteria is dark gray. When the water dries, it turns light gray. Mold and mildew grow much more pronouncedly outdoors where frequent moisture provides the water it needs to thrive. Indoors wood tends not to get wet on a regular basis and hence doesn’t go gray.

Sun Damage

UV light is an energetic wavelength from the sun (photon radiation). This energy breaks down the lignin (the “glue that holds cells together) at a wood’s surface, causing them to separate and deform in a way that diffracts light more than reflecting it. This diffraction is a “blurring” event that makes the wood less colorful to the eye (kind of like a stainless steel mirror versus a glass one). Combined with organic bacteria colonizing its surface, wood then goes “Gray”.

Preserving Natural Wood Color

Can Graying be stopped and the wood be made to preserve its fresh-cut appearance?


  • Cleaning mold and mildew off the surface of the wood will allow the native color of natural wood to be seen by the eye (our Sun Frog Deck Cleaner does this).

  • Also, just as wood looks more colorful when wet with water, treating wood with oils (like with those of our Sealer) similarly improves its color coming to our eyes.

Is Turning Gray Bad For Wood?

Mold and Mildew don't harm the wood. Allowing wood to absorb water frequently and deeply, thereby promoting wood rot fungus to grow, does. Gray is “old”. Colorful is “young”.

Maintaining your wood with an oil-based sealer (and ours contains a broad-spectrum EPA-approved biocide to inhibit surface growth of mold and mildew) diminishes deep water absorption, which prevents wood rot fungus from eating your wood. It also reduces checking (cracking of wood along the grain) which takes place over time by invasive wet/dry (swell/shrink) cycles.


Sealing your wood reduces water caused by wood rot, surface cracking, and mold and mildew. It also beautifies your wood, keeping it looking young, not old!

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