Mold Issue

[KellyVega]

 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.

[Erfar]

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.

[HarrisJacks]

You might want to check for any hidden moisture sources, especially inside the walls or under the flooring. Even a small leak can keep the mould coming back. I had a similar issue in my basement, and after trying dehumidifiers and ventilation tricks, I still had to call a mould removal service to get rid of it completely. They found moisture behind drywall that I wouldn't have spotted on my own.