Years ago the most commonly used finish removers contained sodium or potassium hydroxide. They were fairly effective for removing most types of paints and stains, but due to their extremely high pH, they darkened the wood as a result of alkali staining. If only a section of a wall was exposed to these products, it was almost impossible to obtain a consistent final color over the entire wall. That’s where the use of Oxcon™ came into play and why the use of oxalic acid became known as a Blonding Agent. Oxcon’s low pH helped brighten the wood back to its original color, but if it was only applied to sections of a wall, the resulting color would end up significantly different than the remainder of the wall. The solution to this problem was to apply Oxcon to the entire wall to even out the color of the bare wood and avoid a blotchy appearing finish.
Today’s finish removers are mostly based on organic solvents, and one of their most advantageous features is that they don’t change the color of the wood. When removing an old, tired oil finish, it is not unusual for pressure washing with just water and a mild cleaner like Log Wash™ or Wood ReNew™ to remove any residual pigments and extenders on surfaces directly exposed to the sun and weather. But underneath roof overhangs and shaded areas, the stain may still be in pretty good shape. There is nothing wrong with using S-100™, StripIt® or other organic solvents only on those spots where the finish is still adhering. Since they don’t change the color of the wood, you may not be able to see where these products were used.
Wood cleaners are a somewhat different story since they are designed to both clean and brighten the surface. It is rare that anyone would want to clean a section of a wall without cleaning the entire wall. Once a wall has been cleaned with Log Wash, Cedar Wash™, or Wood ReNew, there is nothing wrong with going back and spot cleaning areas that are still showing signs of dirt or discolorations with the same product.
Log Wash, Cedar Wash, and Wood ReNew are not interchangeable! Never attempt to clean part of a wall with Wood ReNew or Cedar Wash and the rest of the wall with Log Wash. Since Wood ReNewand Cedar Wash are alkaline, and Log Wash is acidic, there is a very good chance a chemical reaction will occur and the resulting color of the wood will differ. If Wood Renew has been used to remove grayed wood on an area of a wall, there is nothing wrong with using Log Wash afterward as along as the Wood Renew is completely rinsed from the wood surface, and it is applied to the entire wall to ensure a uniform color.
Chlorine bleach solutions are not recommended by Perma-Chink Systems and should never be used for spot cleaning. The more the surface is exposed to bleach, the more damage is done to the wood fibers, and the greater the chance that discolorations or adhesion issues will arise sometime in the future.
Oxcon Oxalic Acid
Now that most of the log home industry has followed our lead and stopped recommending chlorine bleach solutions for preparing logs and siding for a finish, the next most problematic surface preparation product that we encounter is oxalic acid. The use of Oxcon oxalic acid should be limited to eliminating inorganic discolorations, such as metallic tannate stains, strap marks, rust stains, and water stains.
Although the label clearly states “Do not spot-treat since this method of application can cause blotching”, there are some steps that can be taken that will allow localized discolorations to be spot treated while avoiding the risk of creating blotches on a wall.
The first step is to make sure the wall has been well cleaned using Wood Renew or Log Wash. Log Wash is preferable since the use of Wood ReNew will occasionally darken the wood a bit. If that occurs, a Log Wash solution (two cups of Log Wash per gallon of clean water) will typically brighten the wood back up. Once the surface is clean, bright and dry, a solution of Oxcon may be applied to those areas that show signs of metallic tannates or water stains. Make sure the Oxcon is diluted to the recommended one part Oxcon to four parts water ratio, and don’t leave it on the wood for more than 10 to 15 minutes. And whatever you do, don’t allow it to dry on the wood.
Now comes the most important step in the process, rinse, rinse, and rinse again. You want to be sure that ALL of the Oxcon is rinsed off of the wood. The best way to verify adequate rinsing is by using pH Strips. Oxcon is fairly strong acid. If the pH of the rinse water on the wall remains below 6.0, it is a good indication that some oxalic acid is still present on the wood.
Once the surface is dry, if the spot treated areas are the same color as the surrounding wood, you are ready to apply the first coat of finish. However if the treated spots stand out from the surrounding wood, you may have to go back and apply Oxcon to the entire wall to even out the color.