I built a 10 x 12 deck with treated lumber, how long before I can put Thompson sealer on it.
You need to wait for the lumber to completely dry. This is because it will be easy for the first coat of the sealer to dry before adding the second and the last one. Get more details on how to apply sealer on http://landscaping.about.com/od/decksandarbors/qt/thomsons_water_seal.htm.
As a professional painter, Thompsons' water sealer can be applied any time,as long as the outside temperature is above 10 degrees C. Also as an added hint Thompsons' needs to be re-applied every year because it eventually evaporates with time.
I used to sell treated lumber and u don't have to put anything on it. Unless your trying to keep it from graying out which it will do anyways over time.