Back story: I am repairing the porch stairs at the front of my house. This is steps from street level up about 36" to a 7 x 12 porch in front of the front door. The house is a 2-level cape, in Massachusetts.
I rebuilt the stairs and hung the upper ends of the PT stringers from the porch joists. All that is fine. Then I bolted a PT 4x4 (using a lap joint) to the bottom of each side stringer to serve as bottom rail posts. Then I got busy with other things and never built the rails and, true to form, the PT posts warped and twisted. Went to build rails and the PT for those warped also. So I gave up on PT, returned everything I hadn't used, and went and sprung for cedar posts and 2x4s for cedar rails.
Now to the question: The cedar posts are lapped over the stringers and bolted with carriage bolts. The bottom of the lapped section is level with the bottom of the stringer, which of course is in ground contact. I want to seal it that it does not absorb water and wick it up into the post. What is the best way to do this? Is copper-based sealer (Cuprinol) good enough? I'm guessing not. Any way to "flash" the bottom of the post? Other ideas?