I’m installing 1×4 quarter-sawn Douglas-fir T&G over 16″ spaced front porch floor joists. To get rid of the slick surface I plan to sand the deck with 120 grit to open the pours. Should I back-paint/seal the wood? The porch is covered and faces south. What sealer, stain or finish should I use?
Normal wood floor stains and sealers won't work very well with both exterior use and south facing exposure. A good product that I use for many different deck surfaces is Sikkens Cetol SRD and Sikkens Cetol Dek. The SRD is a semi-transparent and the DEK has lower amounts of pigment making it more translucent.
Both products will stand up well with a maintenance coat every 2-3 years. It is recommended that all sides and the cut ends be stained prior to installation, back painted. Plus, the first maintenance coat should be within a year, then every 2-3 years.
Test any finish that you choose on the T&G wood before buying.