What Paint to Use for a Butterfly Sign?

I've put up a privacy fence and I want to make a butterfly-shaped sign with the name of my gardens on it. I'll cut the butterfly out of plexiglass. I was thinking of spray painting the background and then painting on details with regular artist oil paint, like Bob Ross paint. If I let it cure a couple of weeks and then polyurethane it, will it hold up to our heat (over 100 degrees during our long summers)? Would the oil paint "melt" or run due to the heat? Is it any different really than oil house paint?

Thank you for any advice.

NancyAnn