Any kind of board that is weatherproof and that will hold paint will work. In my hometown, when doing a 'refacing' of the old buildings, they added a small 'barn quilt' to each of the stores, most of them on the sides of the buildings and most were no more than 3'x3'. They used signboard and I think it's been about 6 years now but the barn quilts still look beautiful.
I've done several for others; I live in town and with no outbuildings there are none here, though I'm tempted to add a small one to my front entrance. Mostly around here they are large and on big outbuildings in the country. As you drive along interstates in KY you will see many barn quilts. If they are simple designs, it's easy enough to block off all but one color, spray paint that color then let it dry, then block that color off and move on to the next color and spray it. Same thing with each color with blocking in between. The paint used then is outdoor spray paint that will adhere to the board. Sort of like stencils, block then paint, then block then paint, one color at a time.
