Sealing a deck in cold weather is actually the best approach for several reasons. Of course, cold is a relative term, and you won't want to work in freezing temperatures or when there is snow on the deck. However, a chilly day in late fall or early spring is ideal.
Sealing a deck in cool weather is an ideal time because sealant dries quickly, but not too quickly. Sealing a deck during the heat of summer puts extra stress on the wood, according to This Old House magazine. Additionally, you're less likely to actively use the deck during cool weather, reducing the risk of foot traffic while the sealer is still wet.
Choose a cool day when temperatures are between 40 and 60 degrees Fahrenheit. A dry, windless day is best for applying sealer. Do not apply sealer if rain or snow is predicted in the next two days, or if humidity levels are high, which will increase drying time.
Before applying a sealer, clean the deck thoroughly with a scrub brush and a cleaner made for decks. Avoid cleaners that contain chlorine bleach, which can damage wood fibers. Oxygenated bleach cleaners usually work better. If you choose to clean the deck with a power washer, do so carefully. Power washers can easily damage the wood. Allow the deck to dry and sand it with a hand sander to remove splinters or loose pieces. Inspect screws and hardware and replace any that are rusted. Sweep the deck to remove sanding debris and apply a sealer with a long roller or brush. Allow it to dry and apply a second coat, if desired.
Once you've cleaned and sealed your deck, keep it looking new through regular maintenance. Promptly remove snow in the winter, using a snow shovel with a plastic or rubber tip. Heavy snow can stress the wood and prolonged exposure to moisture can lead to rot. Sweep the deck frequently in the spring, summer and fall to remove pollen, dirt and leaves, which can encourage mildew growth. Wash it with a hose occasionally, as well.