Scrape any plants, moss, or vines off your deck, using a putty knife.
Sweep large debris and dirt off the deck with a broom.
Use canned air to blow away all smaller debris and dirt from cracks between the boards. Damp dirt and debris can act like food between teeth that contributes to decay. The moisture will soak into the wood and can contribute to wood rot, so you need to blow it all clean.
Spread sealant onto the deck's surface, using a mop. Applying generous amounts of sealant with your mop will ensure the sealant covers the entire surface. Softwoods like pine will absorb more sealant than harder woods like oak, so depending on the wood construction of your deck, you can expect to use one gallon for every 500 to 800 square feet of surface space.
Apply sealant to cracks or vertical beams, using a paintbrush, making sure to apply enough so that the wood looks evenly dark and wet.
Allow the sealant to dry for 48 to 72 hours.
Brush the ultraviolet-resistant polyurethane onto the dried sealant. The UV polyurethane will create a hardened shell that will help the wood withstand foot traffic.