I'll share a bit of wisdom from local experience. We have a barn quilt trail here. A friend of mine, who teaches art, wanted to join in, so she had her students paint a large one on plywood for the side of our school. It came apart in the weather within 2 years. Our barn quilt group paints on metal and they buy a special paint to hold up outdoors. It's worth the extra investment up front if it will last 10 times longer.