I’ve cleaned a lot of decks, sidewalks, roofs and siding. I use my twelve horsepower pressure washer to do so. It develops four thousand pounds pressure at the wand. It, certainly, knocks moss off a roof and beats it out of a concrete sidewalk. Sometimes, however, brute force isn’t enough, or is too much. Even a Home Depot pressure washer can carve a deck or siding. For that reason, I may have to resort to chemicals.
Using chemicals can greatly reduce the work I have to do with my pressure washer. For example, after treating a roof, much of the moss will blow off with just the pressure of a regular hose spray nozzle. Another situation where chemicals can work wonders, especially in combination with a pressure washer, is siding. I am always amazed at how much build up there is of oil and rubber on houses situated with in a few hundred feed of a busy highway.
Below are some chemical solutions to exterior cleaning problems. All have the capability of causing serious bodily harm, or even death, so use protective gear (e.g., eye protection, a respirator, gloves and long sleeves and pants. As well, they can cause property damage, so note cautions and act accordingly.
Chlorine based bleaches are sodium hypochlorite, calcium hypochlorite and dichloroisocyanurate. The first two are typically used in laundry detergents while the last is a swimming pool additive. Any of them can be excellent for killing moss and mold, but do almost nothing for removing it.
Chlorine bleaches can easily damage wood, so must be used with caution. More is not better. Many of us have gotten bleach on our fingers and noted how difficult it is to rinse off. So too it is when using it in projects. It can compromise and otherwise quality paint job, so must be used sparingly and rinsed well.
As noted previously, household bleach and other products containing chlorine-based bleaches should not be mixed with products containing ammonia.
Oxygen type bleaches can be an alternative to chlorine type bleaches. They include disodium peroxydicarbonate, commonly known as sodium percarbonate. It is commonly found in laundry detergents and such. For example, many of us are familiar with “Oxyclean.” Such products probably have fillers added, such as used to be done with Tide and other detergents back in the day. As such, you can buy sodium percarbonate in bulk and use it in place of things like Oxyclean.
Added to water, it forms hydrogen peroxide and sodium carbonate (soda ash), a mild cleanser.
It can remove mildew stains and weathered gray residue from UV (sunlight) damaged wood, but seems to work best in warm water. It may have to sit for a half an hour to accomplish its task.
Oxalic acid is commonly found in paint stores and is used for removing rust stains from fences and decks. It is also used for redwood and cedar to remove tannins and such that deposit on the surface of such woods. As well, it remove rust stains from plumbing fixtures, but may have to be made into a past to adhere to vertical surfaces. Remember that oxalic acid is toxic and should be handled and used with care. If a surface will ever be sanded, it should be rinsed, then rinsed some more to remove all traces.
The pro’s also use a chemical called sodium hydroxide. Sprayed on a roof or concrete drive and allowed to sit for about twenty minutes or a half hour, it can greatly lessen the effort required to remove moss and mold. As previously noted, much of the moss can be removed with a spray nozzle after treatment.
You can apply sodium hydroxide using one of those garden sprayer that attach to the end of a garden hose. The setting for application depends on the strength of the sodium hydroxide used.
Sodium hydroxide isn’t difficult to find. Most hardware and grocery stores carry it. You’d recognize it by the name “Draino.” However, you may have to look at the contents of several variations of the product to find the one containing the chemical. We buy ours in a five gallon container. It’s more concentrated, so the sprayer mixture setting is lower.
When using this chemical, as with others and in addition to personal protection, wet plants down and try to protect surfaces you don’t want affected by the application.
Here is one of many siding cleaning formulas you can use:
3 oz (2/3 cup) TSP
1 oz of washing detergent, like Tide
1 quart of 5% sodium hypochloride, like Clorox
Enough warm water to make one gallon