I have white cedar shingles on my house. 10 years ago when we put
them up, we treated them with a stain/bleach mixture, and they have
weathered to a fairly uniform gray color (not quite Cape Cod, but
that's 500 miles away), which is what we wanted.

The most exposed side of the house is showing a fair amount of
weathering (wood surface eroding), and I would like to slow this down
by applying something. Question is, what? The main thing I would
like to achieve is some water repellency, and I would like not to
create a seriously blotchy effect. The shingles are going to differ
quite a bit in the degree to which they absorb anything I apply.

So, advice?