I'm working on a costume for a con, and my outfit includes a fake robotic arm. I've been piecing the arm together out of craft foam that I painted steel gray. I've been using acrylic paint — the cheap kind you can get for a few dollars a bottle practically anywhere. My experience with paint is kind of limited, so here's where I need help. Is there any sort of top coat I can use over the first layer of paint to protect it and maybe even make it look like it's polished (it's supposed to be metal after all)? At any rate, whatever I use needs to be $10 or less and easily available at Walmart or a hardware store because I live in the middle of nowhere.