We build timber framed pergolas. These pergolas are exposed to the elemenets. Typically we use Western Red Cedar and so the exposed wood holds up quite well. The wood surfaces within the joinery are of concern to me. What flashing techniques are available to protect these wood to wood connections?

I plan to use copper flashing for aesthetic reasons and for longevity.

While reviewing other postings I have seen reference to using a Borate Paste in exposed joints as a preservative. Is this commercially available or home mixed?