If you want to brighten the stain, you could apply another coat to the deck, or you may want to simply seal it with a water repellent by the same manufacturer, or a similar product that won't change the color of the stain. (Check out the manufacturer's Web site for guidance.)
Location is critical to how well a deck survives the extremes of weather. Decks on the north side of a house or in the shade tend not to dry completely, resulting in mildew and the need for more frequent cleaning and resealing. Your decision to go with a stain with UV inhibitors has served you well, since only after four years are you moved to consider renewing it.
There are several staining options, including clear water repellent without UV protection, which lets the deck gray with cracking, or clear with UV protection, which prevents graying. You should seal every year, no matter which you use. Semitransparent and solid stains with sealers come in a variety of hues and let you go as many as three years without restaining, depending on climate and deck location. If you strip a stained deck, you're going to have to restain.
Your deck probably was built with lumber treated with chromated copper arsenate, which is no longer being sold for residential use. Wear protective clothing and cover plants.
Although I've had good luck with bleach and water for mold, mildew, berry and leaf stains, conventional deck washes containing detergent and sodium hypochlorite work well. For rust stains or ground-in dirt and grime, use cleaners containing phosphoric acid, available in home and hardware centers. For tougher oil and grease stains, scrub with a detergent containing a degreasing agent as soon as possible after the stain occurs.
For those interested in reducing maintenance in general, there is composite decking, which now accounts for 5 percent of the $3.5 billion annual decking market and costs about 50 percent more on average than pressure-treated lumber. There are more than 40 materials options for decks, both natural and man-made. These composites don't need to be sealed or stained.
Some quick cleaning tips:
Removing accumulated grime and mildew is best done on cooler, cloudy days that allow the deck to stay wetter longer, and give the cleaner time to work.
You'll need to know the square footage of the deck, including stairs and railings, to figure out how much cleaner and sealer you need. A gallon of stain or sealer typically covers 150 to 300 square feet of deck (commercial deck cleaner typically lists square-foot coverage on the package).
Apply cleaner (available at home centers and online) with a paintbrush or a sprayer. Let the cleaner work into the deck surface for about 15 minutes, then remove the residue with a squeeze mop; dirtier decks may require more than one application. Don't scrub, even if dirt and mildew are really imbedded, to avoid gouging the softwood used in most decking.
Have questions for Alan J. Heavens? E-mail him at email@example.com or write to him at The Inquirer, Box 8263, Philadelphia 19101.