My girlfriend wants to have her grandkids paint a mural on her fence gate.
It's a stockade fence and I think the rough surface will try the kids'
patience. So, I thought about cutting a couple of 4x4 pieces of thin wood,
beveling the edges so rain runs off, priming them, and letting the kids use
those as their "canvasses. A friend at a paint store offered to add vivid
tints to several small batches taken from a gallon of white exterior latex.
Coat the finished masterpieces with polyurethane or something. Attach to
gate. We're already adding a gate roller because it tends to sag a bit. So
extra weight shouldn't be an issue.

Plywood seems like a bad idea for this purpose due to the exposed edges,
even if they're seemingly well protected. I can get 1/4" pieces of pretty
much any common wood. Any suggestions?