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.
Could be tannin staining. Under certain conditions (moisture) some species of wood can stain itself. My log package was delivered that way after spending most of the winter in a log yard out in the weather . Oxalic acid and bleach will not help. I had to pay a company 7K to sand it all off.