We are covering three classes of topical wood protection here - Stains, Sealers, and Polymer wood coatings.
Each is made for the purpose of protecting wood from harsh environments of varying degrees and each has pros and cons.
First, these are the main causes of damage to wood:
Water
Water is absorbed into the wood and washes out the natural wood resins and color. Freezing/thaw cycles can splinter and crack the wood. Also water promotes algae, mold, and mildew growth and rotting. In harsh marine environments, marine boring organisms are probably the largest threat to wood.
Sun
Ultraviolet (UV) rays bleach the wood fibers and breaks down the cell structure.
Mold, Mildew, Algae, Fungus
Wood and moisture are a natural food source for mold, mildew, algae, and fungus. This generally causes a black, green or gray discoloration.
Contamination
Contamination comes in the form of food, dirt, atmospheric pollution, etc. This is a good food source for Mold, Mildew, Algae, and Fungus.
Wood Boring Organisms
Shipworms, Gribbles, and termites are the biggest threats. Shipworms and gribbles lives in wood at the surface level of water and commonly cause the "hour-glass" shape in pilings at the waterline. There are various kinds of termites but, basically, they all eat wood. Pressure treating wood usually eliminates most of the danger of termites because it effectively makes the wood inedible to termites.
Wood Stains - Least Protection
Although they do offer a slim layer of protection, wood stains are not actually considered to be protective coatings but many colored stains do offer protection from the sun's UV rays. Further, fungicides are often added to inhibit mold and mildew growth. Be very clear that the primary purpose of stains is to color wood.
Here is what the Paint Quality Institute Says about Stains:
To understand exterior stains, it's important to recognize that these popular products fall into two broad categories, according to experts at the Paint Quality Institute, a leading authority on the subject. There are "semi-transparent" (sometimes called "transparent") stains, and "solid color" (or "opaque") stains.
Semi-transparent stains are applied in a relatively thin coating that allows both the grain and texture of the underlying wood to show through.
Solid color stains contain more pigment than semi-transparent stains, are of heavier consistency, and go on in thicker coats, which still reveal the texture of the wood, but not the grain. Because they are more highly pigmented, these types of stain have more "hiding" ability, which makes them a better choice for blemished wood, and on projects involving different types of wood.
Both semi-transparent and solid color stains come in oil-based and latex-based formulations. Oil-based exterior stains tend to penetrate wood, while latex stains form a protective film on the surface.
The best brands are Cabot and Wolman.
Wood Sealers - Medium Protection
Sealers are made to protect wood against against moisture, sun, snow, (UV rays), and usually from mold and mildew as well. Using a great sealer can greatly reduce the amount of cracking and checking in your wood and greatly increase the life of your structure.
Microporous, or penetrating, sealers are best to use because they allow moisture to escape so it will not remain in the wood and increase the rate of decay. Essentially, they allow the wood to "breathe" a little.
From what I have seen and read, the best brands of wood sealers are Cabot and Wolman although the most popular brands are Behr and Thompson's. According to an article at www.dcs1.com:
"About the only product that most contract cleaners will agree on is that "Thompsons" is not a product of choice!"
For an easy test to find out if your deck or fence needs to be resealed.
Polymer and Plastic Wood Coatings - Major Protection
Polymer and plastic coatings keep chemicals in the wood and harmful elements completely out of it. Environmental concerns about the chemicals used to pressure treat wood made the use of plastic coatings such as EPL more popular because a plastic coating can completely encapsulate treated wood and the chemicals used to treat it. Polymer coatings such as 21 POLY are usually used in marine environments and are extremely effective at encapsulating wood and protecting it against attacks from marine borers. 21 POLY is so effective pilings coated with it come with a 25 year guarantee even in saltwater. That's a big statement because saltwater is a really harsh environment for wood structures.
If there is a downside to plastic and polymer coatings it might be the difficulty or inconvenience of applying them. Plastic is thick and heavy and easily applied incorrectly leaving an uneven surface. Vinyl coatings usually come in two part components and must be applied before construction using special equipment. They are, however, exceptionally effective for their intended purposes.
There is the overview of the three main classes of topical protective wood products. Best of luck on your projects. Leave comments if you have questions.
