My hubby did some. not that large for a barn....
ours are 20"x20" and are on a thread that I sent in..
http://www.quiltingboard.com/t-31055-1.htm
they turned out really nice and are our home..
He primed the plywood. better grade plywood takes the paint better.......he then drew that pattern and painted. He sprayed them with polyurethane and put a frame around them.
Hope that helps........MaryJane