Cleaning and sealing Outdoor Wood
Wood that is used to construct boat docks, decks, handrails, picnic tables or any other horizontal outdoor surface is subjected to the absolute harshest weathering conditions. Unprotected wood absorbs enormous amounts of ultraviolet (UV) radiation from the sun. UV rays can actually blast apart the lightning that bonds wood fibers together. Rain water and snow melt can pond or pool on flat boards. This water can easily be absorbed by the wood causing it to swell. When the sun or wind dries the wood, it shrinks. The shrinkage forces cause cracking, twisting and warping. Conventional pressure treatment chemicals will inhibit rotting but will not stop these other destructive and unattractive phenomena. New wood needs to be sealed immediately. Water and sunlight begin to destroy it as soon as it is exposed to these elements.
Cleaning a wood surface prior to sealing is very important. The cleaning will remove any dirt, tree sap, algae or mildew. It will also remove the decaying wood fibers destroyed by the UV rays. All of these things prevent the wood sealer from adhering or penetrating into the wood. Do not use chlorine bleach (sodium hypochlorite) to clean your wood! It is a very strong oxidizer that will remove the natural color from your wood. In addition, it is toxic to any vegetation surrounding your wood surfaces.

A better cleaner is sodium percarbonate or oxygen bleach. This product will not remove the natural colors from the wood nor will it harm vegetation. Sodium percarbonate should not be used to clean redwood. It can actually darken the wood. Use oxalic acid instead. I have cleaned cedar with oxygen bleach without darkening it. You may want to experiment yourself. If you feel it darkens the cedar, then use oxalic acid to complete the cleaning job.

When selecting a sealer, try to choose one that is a penetrating water repellent. Look for ones that are formulated using synthetic oils and resins. Some are available that are water based. You can - in some cases - clean and seal your deck the same day! Choose a sealant that is heavily pigmented if possible. The pigments in sealants do a great job of reflecting and absorbing harmful UV rays. Some sealants that you can purchase contain natural oils like linseed oil and tung oil. Both of these oils can do a great job of protecting wood, however the manufacturers often have to add mildewcides and fungicides to the sealants. The natural oils are actually food for mildew and algae! That is why many decks turn black or green in just a matter of 6 - 9 months.

Avoid using sealers that are film forming sealants. Beware of products that contain acrylic resins, varnish resins or alkyd resins. Look on the label for these ingredients. These products do not readily soak into the wood. They leave a film that will eventually peel. If the film peels unevenly from your wood surfaces, it will become a maintenance nightmare. You will have to strip your wood in the future to add additional sealants.

Be sure to follow the instructions on all cleaning and sealant products. Hot, sunny, windy days can be the worst time to apply certain products. Some synthetic sealers will last up to 3 years if you apply them according to instructions. I have a synthetic sealer on my front porch that is 2 years old. It looks practically new. There is no sign of mildew or algae growth whatsoever.

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