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I recently built a garage and have installed log siding on the building to match my home.  The siding I used was sitting in a barn for over 20 years and was somewhat dirty, but otherwise ok.  I recently finished the last side of the building (facing east), which is in an area that gets little sun.

Since the boards only appeared to have some dirt on them from sitting all those years, I opted to clean with oxalic acid. After spraying the oxalic acid onto the siding, I let it sit for about 15 minutes, then rinsed off with water.  I checked the siding the following day and it appeared to be clean and looked good. 2 days later (yesterday) I mixed some Timbor borate to treat the siding.  I began to spray and stopped when I noticed, in some areas there was a blue residue which reminded me of a dried up liquid copper.  It was only in a few areas near the knots and where the wood was somewhat rough (siding was milled by the amish).  At first I though it may have been mold, but it didnt look like typical mold (normally blackish) that I see in the area. When I touched/rubbed it, it came right off.

The wood is red pine and some of the boards do have the blue stain type grain in them.  I am just wondering if the blue stain has something to do with residue, or is it probably mold?

Since I applied the timbor and was intending on staining it when dry, I dont want to wash it down with bleach or something, then re-do the borate if I dont have to. 

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Hi Mike,

If you've gone through the process of using Oxalic Acid, (which is a brightener, not really a cleaner) you've lightened the wood best you can and using bleach can lead to more damage to the woods fibers. Without seeing pictures it's somewhat hard to say what it is exactly, but it could well be "blue stain".  Blue stain discolorations usually run somewhat deep into the wood, but should not be able to be rubbed off.  Can you please post a few pictures so we can get a better idea of the condition?



Hi, Mike.

You might be dealing w/ some chemical interactions between the two products, in combination with some loose wood fibers still being on the surface. If you can post more details about how you rinsed off the wood, if you used anything other than water to neutralize the oxalic acid, what the pH of the wood was after using the oxalic acid, etc., that would be helpful. In addition, pictures are always most helpful. If you can post some of those, hopefully we can help you get down to the bottom of this.

We're a bit stumped, so the more detail you can provide, the better. Is it well water, city water you used? Did you use the same equipment to apply the oxalic acid with the same spray equipment as the Timbor? Any and all details may be useful. You can also email me directly at Between me and our chemists in the lab, we'll hopefully get you an answer.

Thanks! --- Charis w/ Sashco - -

That’s kind of what I was wondering- if it was a chemical reaction of some kind. It seems odd that mold would form so quickly after cleaning. I would have taken pictures but I was spraying the borate at night( only time I could with young children currently taking my free time). I’ll try to take a few later tonight.

Prior to spraying the oxalic acid I did spray some deck cleaner spray and forget type chemical made by jomax on my deck, but I did run about a half gallon of water through the sprayer afterwards to rinse it out.

I rinsed very thoroughly with well water which sometimes has a higher iron content than I’d like.

Thanks for the added details. The higher iron content in the well water might have something to do with it, too. We'll try to piece together everything we know with what you know and figure it out.

Many traditional deck cleaners are also acids (phosphoric or oxalic being the most common). I doubt there was much residue left if you rinsed with water, but there could be some residue that was left, creating more of an interaction.  

Timbor will actually have a slight neutralizing effect on acid cleaners. Really, it does the "final step" after a thorough rinsing or power washing. We're wondering if there might still be some loose wood fibers on the surface after cleaning. The oxalic acid does a good job of cleaning and loosening any oxidized wood fibers. Even sitting in a barn, they'll build up quite a few. If those wood fibers are still on the surface and have some of the cleaner and the borate in them, they could create some funky colors. It's also possible, given that much of this is happening at knots, that you've got some tannin bleed going on. Tannins will often times be forced to the surface during the cleaning process. Some of them are very loose and will wipe right off, while others are deeper in the wood and require sanding. They don't hurt the structure of the wood, so aren't a concern as far as rot or decay.

In any case, hopefully that helps you do a bit more research. I'll watch for those pictures.

Enjoy chasing the little ones! They don't stay little for long.

--- Charis w/ Sashco


Thanks for the advice and help. It seems like every time I ask something here you are very helpful.

The blue residue was there before the borate. It’s just that I noticed it when I started to spray the borate. Now I am hesitant to stain the wood if the stains are from mold since I don’t want anything lingering under the stain. On the other hand it was my last bag of timbor and I don’t want to clean it if I don’t have to which would deactivate the borate andinvolve reapplying the Timbor further delaying things. I was hoping to wrap this up beforevthe temperature drops and we get hammered with another Erie winter here.

And I try my best to enjoy chasing the kiddos but my bossy 3 year old daughter has really been testing me lately. I can’t imagine the fun I will have in the teenage years. I’m sure one day I will miss it though

Actually now that I think of it, the other 3 sides were cleaned with sodium percarbonate and I did not have this happen. The reason I used oxalic acid was I found a container of crystals I had leftover in my garage from work on my house and thought it worked better. 

Well that certainly explains things a bit more. My guess is that you probably have some oxalic acid in the wood still. It might be that you just need to rinse some more. Hit up your local pool supply or hot tub supply place, or a paint store. Get a hold of some pH strips and check the pH of your wood. (Amazon has them, too.) That should tell you when you're close to neutral and have gotten all of the oxalic acid off. Once dry, then you can apply the Timbor. Hopefully that does the trick. Still post some pics, though. We'll try to take a look at them tomorrow.

I have 3 boys - 13, 10, and 3. We call the three-year-old our threenager. It makes all the sense in the world. He and the 13-year-old give us a run for our money some days. Thankfully, the joy they bring far outweighs the frustration, on the whole. :-)

Have a great evening! --- Charis

Another thought: if the blue was there before, it could be tannin staining. The oxalic acid would bring it to the surface and loosen it some, which would make it come off easily with a finger. Rinsing the whole wall some more, then sanding just those those areas may help.

--- Charis

these 3 pictures are of the blueish residue. It was a lot more obvious last night but I rubbed most of it off. 


Attached are 2 pictures of something that looks like mold. This was only on a few boards. Does this look like mold. I’m guessing I shouldn’t stain over this. I didn’t see this last night. Now I am thinking I will was with bleach, tsp and water


Mike - That looks like mold/mildew to us. I had our chemist take a look, too. In addition, I'm seeing some wood fuzzing, likely caused by the combo of cleaning and rinsing. You'll want to sand that off with a 60 grit sandpaper. It'll eventually fall off and take your stain with it, so remove it now.

Hit that with some sodium percarb or a 20% bleach solution, then thoroughly rinse. If that doesn't get rid of it, let us know, but that should do the trick. Check your pH to make sure you're back to neutral, then apply the Timbor. As an aside, you don't need to let the logs dry between cleaning and Timbor application. The water left on the wood helps carry the Timbor into the wood, so dry time isn't necessary. DO let the logs dry before staining. :-)

Hope that helps! Happy working and have a great weekend! --- Charis


Do you think the blue is mold and the black mildew? Yes I am definitely going to clean and re-do the Timbor. The wood isn’t fuzzy it’s just like that from the way it was milled 30 years ago by the Amish and is somewhat rough cut in spots. I think it gives it charm and also helps the stain cling. In spots where it is real bad, I hit with the osborn brush. Thanks for your input and enjoy your weekend.


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