The Log Home Neighborhood

An online log home community for log home enthusiasts.

Our log cabin is located in Southern Vermont on a lake.  I noticed that the northern exposure side, under the balcony has this dark stain appearing; almost black. Not sure what it is.  Would anyone have any suggestions on how to get rid of this? will it hurt the logs? I've tried bleach but it does not appear to have any effect! help.........please

Views: 15657

Reply to This

Replies to This Discussion

Hi there.

If the black has not gone away with bleach, it could mean one of the following:

1) It could be mold/mildew, but it may be underneath the stain.  The only way to get rid of mold/mildew that is growing under the stain is to remove the stain and pretty much start from the beginning.  It's not wise to let mold/mildew remain under the stain.  While it is not a direct cause of rot, it's certainly an early indicator that the wood is getting too wet, and wet wood is prime food for wood-destroying rot.

2) It could be tannin stains.  Tannins are water-soluable chemicals in wood that are brought to the surface when there is excess moisture getting into the wood, or just because they feel like it. (Tannins are like that.)  They don't affect the structure of the wood, only the aesthetics.  Tannins can be removed, but are always under the stain and, again, you'd have to remove the stain and start from the beginning.

3) It could be blue stain.  Blue stain is common in trees that were insect infested before they became a home or cabinets or whatever.  Like tannins, they don't affect the structure, only the look.  But blue stain is nearly impossible to get rid of.  Medical-grade hydrogen peroxide (35% concentration) will lighten it some, but won't get rid of it completely.

TO TEST TO SEE WHICH ONE IT IS: You indicated that you tried bleach.  Did you try a test patch with straight bleach?  If not, do that.  Take a q-tip or cotton ball soaked in straight bleach and apply to some of the black.  If it disappears after 1-3 minutes, you have mold/mildew under your stain.  If it doesn't disappear at all, it's either tannin staining or blue stain.  

Pics will help.  If you can post some pics here, those of us who have seen this will probably be able to tell you what it is and give you some clear direction on how to treat it.

Hope that helps.  Happy December!

-- Charis w/ Sashco - -

Hi Charis,

Thank you for your advise, I appreciate it.  I will be doing the q-tip test the next time I'm up there.

Can I do this test even during the winter months?

Thanks again,


Doing the bleach test in winter is no problem.  Just be sure you use fresh bleach.  When bleach gets old, it loses its effectiveness.

You won't be able to do any of the work necessary until temps get warmer in the spring time, but at least you can find out if it's one or the other, and have your ducks in a row come April or so.  (I suppose you could do the work in winter, but it'd be very expensive...we're talking hanging up plastic sheeting, bringing in propane heaters, etc. etc....)

-- Charis

Charis! Your answer here answered 1 of my questions for the past year...what is that almost "pretty" blue stain!  Thank you! Now im not so worried about it!!


vicki smith.



If I could "like" your comment (Like on FB), I would.  So - like!  Glad I was able to help in a backwards way.  :-)

The funny thing is that many suppliers charge a premium for the pretty blue-stained cabinets or flooring.  Blue stain is caused by the bacteria left behind by insect poo....gross to think about, but makes for some pretty floors & cabinets.

- Charis

What if the logs pass/fail the test.   Did the chlorine test on mine and the black went away.  I had stripped my home 3 years ago and then restained with oil base stain.  When I went to wash it down good this year, you could see areas turning black and they ended up washing off with just a garden hose.  These areas are where I did the test.  Whats the safest best way to strip if I need to?

Bryan, if you were able to wash it off with a garden hose, then it is just mold/mildew growing on the exterior.  Also if it washes off easily, it means it is just on top of the finish and not down into the wood yet, which is a good thing.  What tends to happen with any log home is that dust, pollen, mold spores, etc. land on the tops of the logs and sit there and the mold begins to grow on the dust.  If not addressed, it will begin to break down the finish and eventually the wood.  Every log home should be washed with a mild detergent, soft bristle brush and garden hose at least once a year, more if you continue to have a problem.  If its under the finish, the finish will need to be removed and start over again.


Unfortunately, I am afraid the mildew might be under.  The places that turn darker as I rinsed the home off, were the places that the stain came off as I kept rinsing with just a garden hose.  These are the places that tested positive with the bleach test.

Jason is right - if it's under the stain, you'll have to remove stain and start over. Somehow, the surface was compromised, which allowed the water in.  I'd be happy to talk to you about how that may have happened, so it doesn't happen this time around.  Here are a couple quick recommendations: 


1) This time around, if you can, stick to dry prep.  Stay away from power washing.  You will obviously have to rinse with a cleaner to kill the mold/mildew problem, but stay away from large amounts of water.  Someone without these issues and who lives in a drier climate could easily do power washing, but when the problem is already there, adding more water to it just exacerbates it.  Of course, wait for proper weather.


2) Do some research & get more stain samples to try out.  We did an educational webinar for Log Home Living last year called "How to Choose the Right Stain."  It went through different types of stains (there are more than just 2), and it also discussed types of prep. You can still watch it by going here:


Hope that helps for now.


-- Charis w/ Sashco - -

Thanks, I will definitely watch this.  My thoughts were that when I stripped it, I used a strong chemical stripper due to the type of coating I had on it and I used a large amount of water to rinse and then neutralize.  When I finished stripping, of course the weather wasnt ideal and I had to let it sit for several weeks before staining.   I presume it would be possible that the surface might have been compromised with mildew at this time and I simply sealed it in. 

I did just enough research the first time around to be dangerous and used a coating which was very durable, the sacrifice was that the coating was thick and soft.  This first coating was infiltrated with artillery fungus from wood mulch (that we definitely got rid of asap). 

Thanks for the suggestions,


I wanted to get your thoughts; I just did the bleach test and the dark stain did not go away. It had no effect on it at all?  I'm going to send a picture once I figure out how! 
Thank you- john

Hi John,

If you could send the picture to me at this address;

I would be glad to look at it with my co-workers and see if we can come up with some advice for you. I also have some tech tip pdf's about the different kinds of dark stains that can happen to wood and the steps that you can take to clean, remove, and/or mask them. I can email this information to you if you would like just let me know.

Kevin, PCS Redmond


© 2020   Created by Neighborhood Host.   Powered by

Guide to Log Homes | Advertise | Badges  |  Report an Issue  |  Terms of Service