Any coating that is “clear”, i.e., without pigment, will not provide protection against UV exposure. A finish coating must contain pigment in order to block, absorb or reflect damaging UV rays. Stain pigment protects your deck's surface much like sunblock lotion protects your skin: both prevent the top surface layer from drying out and aging prematurely.
When dry, oil-base products will provide a deeper, shinier finish on the deck surface, while water-base products produce a flat, or matte appearance. Be sure to follow the product label's directions on recommended number of coats, so that you achieve the product's intended finish look.
To protect wood decks or other structures exposed to constant shade, which promotes the growth of mildew, consider using a water-repellent with a preservative. The preservative will give your deck added protection against fungi growth, including mildew, mold, rot and decay organisms.
To protect wood decks or other structures exposed to severe heat and sun exposure, use an acrylic stain with transparent iron oxide or opaque pigments. Stains with this make-up will provide your deck with longer color retention than transparent stains or toners, giving your wood maximum protection against graying and surface degradation.