I want to first address this by stating that the Forest Product Laboratory which is part of the Department of Agriculture is the leading authority when it comes to standards for the industry and Sam Williams and the Joint Coatings Committee have written many of the standards today. Technology and processes are moving forward at such a fast rate that it is hard to create a standard at one point in time and then apply it down the road. There has never been a standard for pressure washing by this group and one that Sam Williams did want at one point. He is retired and to my knowledge this was never accomplished. The WCRLA has come out with a policy to "not use a pressure washer" because of the overall damage that is done when using this equipment. Their reasoning is that if they discourage the use then it will get rid of the damage. This, in my opinion, is absurd because anyone with knowledge of the equipment and use of a nozzle chart one can use a pressure washer professionally and responsibly by using the equipment as nothing more than a giant rinsing tool which saves time and labor. Another advantage to this equipment is that basic chemicals left to long on wood without rinsing and/or neutralizing can burn or damage the wood. Chemicals that are too strong will cause wood to fuzz up, specifically softwoods like cedar. Using a pressure washer one can rinse surfaces very quickly. Personally, I feel that educating people on exactly how to use a pressure washer for wood restoration would be more responsible than just saying that because most of the damage done to cedar involves the use of a pressure washer. My guess would be that ANY homeowner or contractor that uses a pressure washer for the first time without the right knowledge will cause damage to cedar. There is a lot of misinformation out there and specifically on You Tube videos that if a person watched some of these would cause extensive damage to their cedar sided home. In regards to the links mentioned above the one from Texas I would not agree with in it's totality either as it recommends a 15 degree tip on cedar which I am not aware of any "wood" guy that uses a 15 degree tip. Again personally, I do not recommend anything except a 40 degree tip and then a variety of 40 degree nozzles with different orifices to regulate pressure up or down. I also feel that depending on how many coats of product is on the wood surface you will use a little more pressure but not much more. The idea with wood is the least amount of chemical and pressure to get the job done all the while relying on dwell times.
Regarding bleach and other chemicals in wood restoration the standard I recognize is the one by the Forest Products Laboratory that recommends a generic solution using household bleach
http://www.mchd.com/pdf/woodpr.pdf. You may ask why they recommend this? It is because they have done extensive testing and found that 3%-3.5% bleach effectively killed mold and mildew on the wood surfaces for coating. There was no reason to go to a higher dilution rate or a stronger product. This is not one of those situations where because 5% household bleach is recommended that using 12.5% or 3:2 is better. Now with that being stated I do not know anyone in particulars methods or products so if you are using 12.5% chlorine bleach in your solution and by the time your solution is mixed and applied the actual percentage applied to a surface is unknown to me. I am only speaking regarding testing and results on the matter.
Regarding cleaning wood vs. restoring wood in the links above or the Forest Products Laboratory Standard applying a preservative is best for the longevity and extending the life of the wood. What is proven is that if you are applying a basic chemical to the wood you are changing the ph in the wood. This one reason why wood brighteners are used, which is to neutralize the surfaces for coating. A wood surface and coating may be adversely affected by a surface that is high in ph and not neutralized or rinsed thoroughly.
I think there is a lot of work that needs to be done amongst folks to get to common ground if that can take place. I unfortunately think there are going to be two different camps on this at the end of the day. I will end stating that chemicals and the products we use can work wonders and help us achieve great results the problem, like anything else, is when they are misused and damage occurs. This is where we should work together in coming up with solutions that in the end do not cause damage and any type of regulation like the WRCLA has come out with regarding the use of a pressure washer.
http://www.mchd.com/pdf/woodpr.pdf
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