I put my wood deck to bed for the season about a month ago. Suffice it to say I was really upset. Back in the spring I went to great lengths to clean the deck and reseal it. I purchased an expensive semi-transparent sealer that did have a small amount of pigment in it. However, after several months, the wood started to gray and then the mildew set in. Now I have to start over when spring arrives. This time I want to do it right. What advice can you offer when purchasing wood deck sealers?

— Brandy S., Albany, Ga.

This is no consolation, but you’re in a large group of consumers that are deeply disappointed with the performance of their deck sealers. I’ve been interested in this topic for years as a contractor, but I’ve also been on the short end of the stick several times as sealers have failed on my own wood deck surfaces.

Wood deck sealers come in all flavors. There are water-based sealants as well as a few oil-based products. Some are heavily pigmented while others have just a hint of pigment. Many homeowners love the look of the lightly pigmented products, as they tend to show off the wood’s natural color. The trouble is these nearly clear sealers and water repellents are ill equipped, for the most part, to deal with the harsh ultraviolet (UV) rays of the sun.

The best analogy I can offer is what happens to me every spring. Without fail, I go outdoors on the first balmy spring day and shed my shirt. I really enjoy the warmth of the infrared rays of the sun on my pasty white skin that has been hidden all winter from any sunlight. While I’m basking like a seal on a rock, the invisible UV rays are causing sunburn.

You can prevent sunburn by using sunscreen. In the same way, the pigments and other nano particles in wood sealers act to block damaging UV rays. Some sealers simply don’t have enough pigment to block the UV rays of the sun that turn wood gray. Heavily pigmented sealers and water repellents that are deep in color are akin to applying a sunscreen to your deck that might have an SPF value of 100 or higher.

Last year, I decided to do an extensive side-by-side test of 16 wood sealers. Some were heavily pigmented and others were nearly clear. I carefully followed the instructions for cleaning the wood and applying the products. I created two sets of wood samples, each containing treated lumber and cedar. Both sets were sealed at the same time inside a garage out of the sun. The samples were allowed to dry.

One set never left the garage. Once dry, I boxed up one set and set them aside so they could be the control samples. The other set was attached to pieces of wood to simulate how they would be applied to a real deck.

These samples were then taken down to my boat dock, where they were punished by the sun from sunrise until mid-afternoon. I can’t think of a better way to test deck sealers than a dock where the wood has no place to hide from the sun.

The shocking thing — although you will not be surprised based on your experience — is that some of the finishes began to fail within a month. What’s more, the sealers and water repellents on the cedar performed worse than they did on pieces of treated lumber.

Some of the cedar samples developed severe mold and mildew even though they were inches away from the treated lumber samples. I suspect the copper used to make the treated lumber rot resistant was leaching through the sealers. Copper is a natural biocide.

The sealers that have medium to dark pigment are doing the best so far. My test has been going on for seven months, so I’ll have more data as we approach summer. But suffice it to say that the lightly pigmented products on cedar already have failed my test.

Tim Carter is a columnist for Tribune Media Services. Contact him through his Web site, www.askthebuilder.com.