Mold Issue

[KWyatt]

I had a maple tree in my backyard cut down and decided to have it milled and dried so I could make a pair of live-edge picnic tables. The wood was dried to 8%. I know the sun and weather will be harsh on the tables, so I used a marine-grade halcyon varnish, applying 5 thin coats with plans to apply new coats every two years.

My problem is within a couple of months I have mold growing along the edges of the tables. The mold is below the varnish, it is not a surface problem. I have no idea why it has developed or what to do about it. Has anyone else seen something like this before, and more importantly, know what to do about it?

One suggestion has been to sand it back down to bare wood, apply a coat of epoxy, and then reapply the varnish. That is a lot of work, which I can do, but want to have a reasonable level of confidence it will solve the problem before I spend that amount of time and money.

Suggestions?

I could not figure out how to insert images of the tables and mold issue into the post, so here are links to images I have on Imgur.

Picnic Tables

Mold Issue

More Mold Issue

[wtnhighlander]

This is a common problem with most wood species that aren't naturally resistant to fungal growth.

Outdoor wood items are exposed to harsh conditions, which tend to break down any finish. Once the tiniest fissures break the surface, moisture and mold do their thing. Completely refinishing with marine-grade products might make it last somewhat longer. These products form a more elastic coating that stretches and moves with the wood, to some degree. But beware, the tiniest break will allow the process to start over. One of our members in Florida finally gave up the fight, and replaced his oak front door with a tropical wood that offered natural resistance to mold.

[Tom King]

Yes, unfortunately that is not unexpected.

[KWyatt]

On 2/28/2024 at 7:35 PM, wtnhighlander said:

Once the tiniest fissures break the surface, moisture and mold do their thing. Completely refinishing with marine-grade products might make it last somewhat longer. 

What strikes me as odd, though, is that both tables started to show the same problems at the same time. I also have a small patio table at the firepit that I made with an offcut of the tree that I finished the same way and it also started doing the same thing. I used Total Boat Haylcon, which is a marine-grade varnish, and sprayed 5 thin coats on all 3 tables. Seems unlikely that all three would have a break in the surface that quickly and all three would mold in the same way.

[Mark J]

Maple has high sugar content and is more susceptible to mold.  Another thought is that the slabs may have had more internal moisture than you thought.  Moisture meters can't read too deeply.  Are the slabs very thick?

[KWyatt]

The slabs were 8/4 and I kept everything as thick as possible to make a solid table. The sawyer has iDry kilns and good meters to bring everything down to the 8 to 10% range. 

[roughsawn]

After my 9/4 and 10/4 oak slabs are down to 7-1/2 or 8%, I resaw them. The inside of the slabs routinely jump up to 10 to 11% moisture. It usually takes a week of drying/acclimating in the shop to get moisture down to readings that are acceptable to me.

[wtnhighlander]

From the photos, it appears the mold grew most quickly along the live edges. First, the corner transition from face to edge creates surface tension in the applied finish the leaves it thinner and weaker there, than on the flat surface, so any moisture ingress is likely to start there.  Second, as @Mark J pointed out, maple has a high sugar content, which is concentrated in the sapwood that makes up the 'live' edges. So, mold appears first in those locations.

[KWyatt]

If I make the decision to scrap the tables and reclaim the wood, I can sand it back down to bare wood, but should I treat it with something to make sure to kill all of the mold spores? I certainly don't want the mold to come back in future projects or to spread to other wood in my stacks, but I would hate to think all of the wood is a total loss.

[wtnhighlander]

I don't think a chemical treatment is really needed in that case, unless you use the lumber in another outdoor project. Then it might delay or reduce new mold growth.