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.
For an outdoor wood deck, the best finish penetrating is a semi-transparent stain. The best prep is using proper cleaners, strippers, and brighteners. Sanding a deck reduces a stain’s ability to soak into the wood grain. If you sand your deck to a completely smooth finish, the stain will not soak in, and peeling may occur. For this reason, it is best to leave the surface slightly rough with some natural texture for the best results.
Remember that your deck is not a hardwood floor. It should not have a shine or be over-sanded.
Do you have any input on this subject? Comment below.