I live in Luskville QC, a little town 30 miles from Ottawa in Pontiac county.
I sometime use washes of paint. The McCabe sawmill project, all the walls were sprayed with a light gray first, then spray or brush on the main color .
After, take fine sandpaper and sand off some of the paint.I have no tried and true system on weathering. What ever my brain tells me how that would look, is the way I try to make it look. I do read books on weathering to get ideas.
I take pictures of tin roofs, old buildings, logs, trees, landscapes, etc.
Have fun Bill