Stained wood outdoors

In the past, I've only used stained wood for temporary outdoor signs. Now I'm reconsidering that idea. Before, I would buy or make my own stain, from thinned down paint. I would apply it, then wipe it off, which does a terrific job of showing the grain in the wood. Then I would varnish it, to give it that rich wet look, which the project wouldn't be complete without. However, the combination of wood and varnish was questionable, in terms of durability.
Last year, in early October, I stained and clear coated the plywood panels on my son's horse trailer. This was new plywood, which replaced the existing plywood. I made a stain from brown oil based paint, let it dry, then used an automotive grade clear urethane as a top coat. It's been exposed for 10 months, which is 1 winter and one summer, and is holding up well. If I recall, there is only 1 coat of clear, if I do it again, 2 or 3 coats would be better. Here's a picture:
Plywood outdoors
This next sign was made years ago, it's hand lettered and has a varnish top coat. It has been indoors the whole time, but I wanted to show what a yellow stain looks like.
Open Sign
Finally, here's a faux old sign, painted old barn board:
Sign as decor centerpiece