[sanddrag]

It won't be as durable as anodizing or powder coating but you might even want to try your hand at painting if you can afford the cure time. A little detail spray gun is cheap if you have a sufficient compressor to run it. An enamel paint like Rustoleum or some sort of farm and implement paint can be rather durable, and can lay down pretty nice when thinned and sprayed. Of course, all the success is in the prep work. With bumpers nowadays, a lot of parts of the robot never see any real contact, so paint may work out.