How to stop mildew forming under new varnish

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Forum topic by albachippie posted 11-13-2014 10:42 PM 850 views 0 times favorited 2 replies Add to Favorites Watch
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772 posts in 3062 days

11-13-2014 10:42 PM

Topic tags/keywords: question oak finishing

Hi all, I’m in the middle of making an oak table for a customer. Last night I applied the first coat of varnish to some of the components. All was good. But, this morning I went to the shop to check how things looked and found several areas with what looked like mildew marks under the newly applied varnish. Very disappointing!

A few questions please.

Firstly, could this be because the moisture content was too high in the oak? My workshop is pretty damp, especially at this time of year, and I’ve had the oak stored for a few months. I did test for moisture, and it didn’t seem that high.
Secondly, how do I get rid of marks? They will sand out when I sand back to the bare timber, but will they return?
Thirdly, do I need to treat with an antifungal solution prior to varnish?

Any advice greatly appreciated,


-- Garry fae Bonnie Scotland

2 replies so far

View bobasaurus's profile


3486 posts in 3211 days

#1 posted 11-13-2014 10:47 PM

Odd that anything living could grow under a varnish… was it water based? Is it maybe just excess moisture or other surface contamination reacting with the finish? You could try to sand it mostly off until the marks disappear, then apply a coat of dewaxed shellac to seal it before doing any further finishing.

-- Allen, Colorado (Instagram @bobasaurus_woodworking)

View albachippie's profile


772 posts in 3062 days

#2 posted 11-13-2014 11:04 PM

Thanks for your comment Allen. It is a water based varnish i was using. Apparently it’s not that uncommon, just not many solutions that I can find online. I had thought of some kind of sanding sealer. Maybe worth a try.


-- Garry fae Bonnie Scotland

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