The Basics

Q. How does water damage wood?
When wood absorbs water, it swells. As the wood dries, it contracts. These repeated expanding and contracting cycles cause small cracks which grow and expand over time. In cold climates, this cycle is exacerbated by freezing and thawing. Excessive moisture content in wood also supports fungi that cause wood rot.

Q. Does sunlight damage wood?
Sunlight will turn the upper layer of wood cells silver over time, but it will not structurally damage the wood. It will also affect stains and finishes over time, which can then open the wood up to damage from water.

Q. I’ve just built a new deck or fence. I’ve heard it needs to “season” – or be left untreated – for a while. Is this true?
The longest you should ever wait before treating new pressure treated wood is 30 days. (This is because sometimes the lumber can still be damp from the pressure treating process). Otherwise, the wood will begin to fade and sustain damage – and this can happen quicker than you would think. Check the label on the waterproofer you want to use for guidance. Some, like Thompson’s® WaterSeal® Waterproofing Wood Protector, recommend waiting 30 days before application on new pressure treated lumber. Other products, like Thompson’s® WaterSeal® Waterproofing Stain, can be applied immediately to new pressure treated lumber. New cedar and redwood can always be treated immediately.


Q. Why do you suggest cleaning brand-new decking and fencing wood?
Even new wood can have dirt or natural sap/tannins on it, or something called mill glaze. Mill glaze is formed when the friction of planning machinery heats the existing sap in the wood, momentarily liquefying it. Once the surface cools, the sap re-hardens as a glossy sheen, which can interfere with a new coating. Anything that is on the wood can keep the new waterproof coating from being properly absorbed, and so the wood should be cleaned first.

Q. How long will it take to clean my deck? How long should I scrub?
There is no hard and fast answer on how long a project will take. The more dirt or old coating on a deck, the more thorough you will need to be in cleaning. It takes me about 3 hours to completely clean my multi-level, 400 sq. ft. deck, (with a set of stairs but limited railings/banisters) and I don’t use a power washer.
Again, there is no rule on how long to scrub each area of the deck. If an area has just light dirt or gray color, a few passes with the scrub brush may be enough. For more dirt or an old coating, you may want to spend 30 seconds on each area. But – you cannot “over scrub,” and you can always re-apply cleaner, if needed. 

Q. Should I use a power washer to clean my deck?
The thing about power washers is that it is easy to damage the wood if you’re not careful. You can cause “feathering,” which is a fuzzy-looking surface. You can even “carve” into the wood if the pressure is extreme. If you do choose to use a power washer, begin with the ready-to-use cleaner, and use the power washer for extra rinsing muscle, keeping it at 1,200-1,500 PSI. The use of a cleaner is important: while the power washer can get rid of the stuff you can see, it leaves behind mold, mildew, and algae spores. The cleaner will help to kill those spores so that they don’t interfere with the new deck coating.

Q. How will I know if the wood is clean enough?
It may be hard to tell while the wood is wet. Allow it to dry at least partially - there should be no more gray color, mildew or old coating. You should see mostly a natural, brown wood color. Just remember that old coating or dirt left on the deck will keep the new coating from being absorbed by the wood.

Q. How will I know if I've rinsed the deck adequately?
You should not see any more “sudsing” once the cleaner is rinsed from the wood.

About Jeff Wilson

As a narrator, host and actor, Jeff Wilson has appeared in nearly 200 episodes on HGTV and the diy Network in addition to many hundreds of commercials, corporate films, features, and made-for-TV movies. For almost 10 years, Jeff, has partnered with Thompson's® WaterSeal® to help people create beautiful outdoor spaces and maintain them as easily as possible. He is currently the host of HGTV Dream Home, HGTV Green Home, and diy Network Blog Cabin Video series.