How Wood Sealers Work
Wood sealers work by penetrating the pores in the wood and preventing water and other substances from causing rot. However, wood decks and other outdoor wood pieces sit out in the sun or can stay damp for long periods of time causing sealer to break down. If the wood is dirty or has algae and mold growing on it the sealer cannot penetrate. It is useless to apply a wood sealer to dirty wood.
On new wood or wood that is going to be used for inside projects, the sealers not only protect the wood but they aid in the overall appearance of the wood by slowing the staining process creating a much more even tone. They also help to blend in the woods grain pattern.
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Paint primers not only seal the wood but hide knots, grains and stains. They can be latex based, alkyd based or shellac based. These primers are a much thicker sealant that allows you to paint over them and have a uniform surface.
Cleaning the Wood
If you are just cleaning dust or saw dust from a new piece of wood than tacky paper is enough. But if you are cleaning wood that has been outdoors such as a deck or picnic table, than much more cleaning is needed. To clean the wood good quality oxygen bleach should be mixed in some warm water and brushed on to the surface. Using a long handled brush and scrubbing the surface should be enough to take off algae, mold and leaf debris. Once the surface is cleaned it should be checked for damage. If there is any significant water damage or rot the piece should be replaced. The wood should be allowed to dry well before any sealant is applied.
Applying the Sealer
Wood sealers can be applied with a brush, roller or sponge. Every sealer is different and the manufacturer's directions should be followed. When you apply the sealer it will soak into the wood rather quickly. The only sealer that you will be able to directly see where you put it is the primer types. This means you should have a pattern you follow so you know that you have covered the entire piece of wood. Most woods will need at least two coats of sealer to properly soak into the wood and provide the best protection.