Answer your questions here.

Should I paint or stain my wood deck?
A. You should certainly protect your deck from unnecessary moisture intrusion and the harmful effects of the sun's ultra-violet rays. Use of a penetrating stain, containing water repellant ingredients in most cases will protect your deck best. You may also consider using a clear water repellant, but this will need more frequent maintenance. The pigment in the stain will absorb the ultra-violet rays from the sun and protect the wood fibers from degradation. In most cases a stain will need maintenance every 2-4 years while the clear sealers require maintenance every 1-2 years.
Prior to finishing the deck it is important to wash the deck thoroughly to remove dead wood fibers, dirt, and any mildew spores. It is important to note that inactive mildew spores can be present and not be visible, but still should be treated.