Wood cleaners and soaps in general are typically alkaline (basic) in nature. Most alkaline cleaners contain some “free” caustic that is difficult to rinse out completely and can darken wood. There is still a bit of it on and in the wood after the surface has been power washed. You can tell because the surface feels soapy, even after rinsing. If the surface is not neutralized the durability of the finish coat can be affected. The pH of the surface should be close to what wood is naturally. Most woods are slightly acidic.
The best way to quickly and easily neutralize alkaline residue is to apply a mild acidic solution after washing.
Brighteners/Acids To Choose From
Typical acids used for neutralizing are oxalic, citric, phosphoric, and sulfamic, and acetic (vinegar). Each one has plus and minus characteristics.
Oxalic acid is the most widely used brightener in the wood brightening business. A solution of oxalic acid instantly neutralizes the wood from alkaline to slightly acidic. It instantly “brightens” the woods appearance. Oxalic will etch glass and aluminum. When oxalic acid mixes with an alkaline on the wood surface and is allowed to dry it forms calcium oxalate. Calcium oxalate does not rinse very well at all. For the person cleaning the interior of a log home this can be a real problem. When the interior of the log home is finished the calcium oxalate can come right through the finish. You won’t know it is there until the finish is applied. Oxalic acid is also the only one in this group that is labeled as a poison.
Liquid phosphoric 30% concentrate mixes well with water and does an excellent job of neutralizing and brightening. It rinses very well. Phosphoric acid can etch metal.
Citric, though not as powerful as oxalic, does neutralize and brighten. It mixes and rinses easily with water. Citric won’t etch glass or metal. Citric may not neutralize highly alkaline surfaces as well as oxalic or phosphoric. Citric is somewhat expensive.
Sulfamic acid neutralizes and brightens well. The crystals dissolve quickly in water. Sulfamic rinses well and is inexpensive. It won’t etch glass or metal. Sulfamic acid can turn pitch pine yellow if it not applied onto damp wood.
Acetic acid (found in vinegar) is one of the weakest acids. Homeowners who do wash their decks frequently (which is a good idea) can use a pint of vinegar per 5 gallons of water as a neutralizer. Professional applicators would not appreciate the weaker strength.
Bleach was always the one product people turned to when they wanted to zap mold, mildew or algae. Bleach works well to zap the plant or visible growth when used correctly. The problem is that when the mold and mildew growth out number the active chlorine the mold and mildew win the war. When the growth isn’t completely killed off it will eventually return. Spores (seeds) are another battle. Bleach is very sun sensitive and runs out of strength after being bathed in sunlight for an hour or so. Therefore it can’t leave any residual materials behind that will zap the spores (seeds) when they sprout.
So just using bleach over a lot of mold and mildew is no guarantee that the problem is solved.
The best method of dealing with mold, mildew, and even algae is to apply the cleaner, power wash to remove the majority of the mold, mildew, and algae, and then brighten and disinfect. By cleaning first and then disinfecting your efforts are more than a match for the remaining mold and mildew.
So how do you brighten and sanitize with something that is mold/mildew/algae specific, stable enough to withstand the sunlight, and be safe to use if used correctly?
There are some very interesting, safer alternatives to bleach. They are called Quatirnary Ammoniums or various types of Ammonium Chlorides. Ammonium derivatives are the newest, safest, and cheapest way to eradicate mold, mildew, algae, and more. Alkaline Copper Quats will replace Calcium Chromate Arsenate (CCA) in pressure treated wood by 2004 — they are that good. So after you have soaped, washed, brightened, and lightly sanded mix the Ammonium derivative of your choice (and availability) and spray on a solution of the disinfectant on a day or so before you put the finish on. No spores will survive to come back later and haunt you. Always use these types of highly specialized disinfectants according to directions.
How I (You) Did It
Mix and apply the cleaner. Power wash carefully. Mix the brightener. Apply the mild acidic solution. Rinse again. Allow the wood to dry. Do any sanding that might be required. Mix the ammonium chloride or “Quat” disinfectant according to directions on the label. Apply the solution to the wood and leave it there.