Mould growing after using water based filler on wood floor

[Richard Burns]

Hi everyone,

I wonder if anyone can offer any advise to a very disheartened man right now

I have recently sanded down the original parquet flooring in my 1960s house, all looked fantastic and then I used a water based joint and gap filler. When I came to it the following day it's got mould everywhere.

I contacted the manufacturers and they told me mould spores can lay dormant in the oak flooring and the filler has probably set it off.

I have since re sanded and it looked great again, however it appears some areas of mould are trying to make there way back.

Can anyone offer some advise on how to stop this?
I was planning to use Osmo Polyx on it, but don't want to waste product if mould is going to be visible or keep growing back.

Any and all ideas welcome

Rich

 

[HOJ]

Water and Oak aren't good bed fellows, tends to bring out the tanins, my other question would be, what tooling did you use to apply the filler?

 

[Sgian Dubh]

Moulds like moisture. It's likely your joint/gap filler maker has a point: fungal spores can remain dormant for long periods in adverse conditions waiting for the right conditions for growth, i.e., generally warmth and water, so it seems likely you supplied the water when you did the filling.

Time and dry conditions will reduce the water content of the introduced filler (and wood) through evaporation, and when the filler and the surrounding wood are dry enough the fungal activity will stop: dry rot safe is considered to be at or below 20% moisture content, but most fungi need a higher moisture content than this figure to thrive. At that point (halted fungal activity) you should be able to deal with the visible discolouration which is the result of that fungal activity, e.g., sanding and/or oxalic acid treatment, and maybe some dying and/or staining.

I suppose there is a need on your part to ensure there isn't a historical and still active water source in and around the floor leading to raised wood moisture content levels, but from what you've said I guess that isn't the case. Slainte.

 

[Richard Burns]

Water and Oak aren't good bed fellows, tends to bring out the tanins, my other question would be, what tooling did you use to apply the filler?

I used the dust from the sanding mixed with the filler and applied with a large plastic caulking blade.

Forgive my ignorance but what are tanins?

Regards

 

[Dee J]

Second thread on this subject? What makes you think it's mould other than the appearance.
Anyway.... Tannins? Tannic acid, naturally present in oak. Can react with iron in the presence of water to produce a black staining.
I note you used oak sanding dust in your filler mix. If that dust was contaminated with any iron/steel dust - say from sanding over remains of staples, tacks and other random bits found in old floors - then you've set up the classic situation for black staining to occur.
While oxalic acid can lighten such staining, if my suggested diagnosis is correct, it may reccur because the iron/steel is still there in the mix.

 

[Sgian Dubh]

I used the dust from the sanding mixed with the filler and applied with a large plastic caulking blade.
Forgive my ignorance but what are tanins?

Based upon the additional information you've supplied I suspect, like Dee J and HOJ, the black marks are caused by the chemical reaction that occurs between water, iron, and the tannin in the wood rather than mould growth. You mention mixing oak dust in with the filler, so that's a source of the tannin, there's water in the filler, and I suspect there's been a source of iron involved somewhere, e.g., screwdriver, chisel, mixing vessel, etc.

How to fix? Not sure, but it might involve scraping some of the existing filler out and refilling but omitting one of those three key components of iron, water and the tannin rich oak, or you might just be able to get away with a bit of light dying or staining without removing anything. If the black marks or spots are only visible under close scrutiny, i.e., your face within inches of the marks to see them, then maybe you don't need to do anything except go straight to applying a finish.

The tannin, by the way, is the same stuff that is found in tea leaves and which hot water causes to leach out to blacken the teapot, the mug, and even your teeth. Slainte.