I'd probably also seal it after it's painted with a clear coat. I'd imagine if this banner is hanging out in sunlight a lot, it'll get a fair amount of UV damage. maybe like what they use for outdoor furniture.
Discussion
The banner is pretty big and the lettering will be too, so it's very visible from the street if I hang it as part of my backdrop (which is inside my tent, and in the shade). Only in desperation would I hang it outside. But if I decide to do that, would that be an acrylic clear coat?
yeah, I'd spray it instead of painting it on though. you want it to be thin so it can stay flexible. maybe there's an acrylic clear coat made for fabrics?