Updated February 2020
Your Deck is not a hardwood floor and should not be shiny or excessively sanded smooth.
We have had people write in and ask if they should apply polyurethane to an outdoor deck. The notion of having a beautiful shiny, smooth finish like an indoor wood floor on your outdoor deck may seem appealing to some of you, but it is not practical. You should not apply polyurethane to an outdoor deck surface for several reasons. Rain and harsh UV rays from the sun will penetrate the polyurethane cause it to fail quickly, leaving the wood susceptible to weathering. The polyurethane will eventually blister and peel on exterior wood. Those who have experienced a peeling or blistering deck knows how much of a nightmare it is to fix, as heavy sanding will be involved in the removal.