Higgs Boson wrote:

Are you talking horizontal, or 90 degrees where backsplash meets
counter?


Good point. I meant horizontal.

For the backsplash, I butted the horizontal and vertical pieces as tightly
as I could. Then I covered the intersection with small quarter-round,
stained to match and covered with spar varnish. Before placing, the joint
was suitably sealed with silicon caulk.

So far, that methodology seems to have been adequate. I can post a picture
if the description is not sufficient.