Wood decks, fences and siding are exposed to high levels of stress from the severe weather conditions that shrink and swell the wood. Without proper maintenance,wood decks can develop problems such as checks and cracks, raised grain, and mildew, thus increasing the risk of decay and insect attack.

Applying stains, sealers and other finishes to wood will minimize the problems of cracking, raised grain, and mildew growth. There are 3 primary types of useful sealants and they differ in how they work, some are labeled as water repellants (WR) and others as water repellant preservatives (WRP). The only difference between a water repellant (WR) and a water repellant preservative (WRP) is the addition of a mildewcide or preservative to the formulation.

Open Finishes: These are purely "color coats", and provide limited or no protection from absorption of water or from chemical attack. Although they allow for escape of absorbed moisture, they're characteristically soft, have minimal surface build, offer little or no protection against wear, and no protection against spills. Least expensive, least effective.  Most of the $8 -$10 per gallon "latex semi-transparent" stains fall into this category.  90% water and the balance is color and a little binder to hold it together.

Closed: Typically hard and glossy. The closed finishes are especially ill-suited for exterior wood such as decks and siding. They repel water well, and may in some cases be chemical-resistant, but because their upper surfaces are totally closed they trap moisture in the wood and will contribute to premature rot or decay. Hard finishes generally lack elasticity, penetrate the least, and may loosen from the surface prematurely. Varnish & polyurethane's fall into this category.  Least desirable finish type.

Microporous: Also referred to as "penetrating sealers" are the "perfect" finishes. Their open quality is critically important to the long-term preservation of wood, as it allows the escape of moisture which may have entered from the underside of shakes, decking, siding, etc. - it allows the wood to "breathe" with just enough film on the surface to prevent dire and other contaminants to penetrate. The best are acrylic blends.  Water base,easy to apply and cleanup.  #1 choice.

Note: Clear liquid silicone sealants and blends containing wax have frequently been proven to last for many years, but are completely closed. They are cheap to produce, bead water but are highly likely to actually foster mildew growth beneath their surfaces! They're nearly impossible to remove, can't be over coated, and should be avoided for exterior applications. Read The Label!

When buying a clear sealer read what is in the can, you do not have to be a chemist to understand what you are buying. One of the most heavily advertised "wood sealers" is 95% mineral spirits and 5% alkyd resin. Those "specials" you see advertised, "Latex porch and deck stain, $7.99 per gallon" Read the label and see how much water you are buying, 5-10% solids is not going to last very long. Pay a little more and buy a good quality high resin content sealer.