I wonder if sealing on an outdoor frame would be a good idea? Let's use the post to sill tenon as an example, if you coated the mortice and the tenon, could the coating on the tenon act as a bucket and hold water in causing premature deterioration of the tenon? I suspect it would be much slower to dry out after a good winters soaking. I think it would be interesting to look at frames 150 years from now to see how thing went, with all the newfangled devices thrown at them, from metal connector, curved members, glues used, air tight houses and such.
I don't like using sills in completely open frames for this reason. We did a frame that was infilled and it had sills -- we drilled drainage holes through the bottom at every mortise. I'm sure they were filled up by dirt daubers or spiders or something else within a year or two, but hopefully they are doing some good.
I agree with the others that say sealing joinery can be helpful in some circumstances. You wouldn't want to cut an oak frame in Georgia during summer without sealing the end grain.
