I think people use a 50/50 mix of white latex paint and water on the SW half of the tree in the fall. There's other things you can use as well if you search online....tree white wash, lime wash, white milk based paint, etc.
You can use white guards, but voles can wiggle in there and gnaw right up the tree. I have a maple out back that I noticed has a large crack right down its south facing side.