For small nicks, nail polish is good. And comes in most every color you might need or want. Also touch up paint from a auto parts store, or better yet a auto paint supply store. Especially if it's a larger area that needs paint. Paint from a hobby store for plastic models cars and airplanes too.

Or just not worry about it. I think nicks and scratches add character. Sometimes they are a conversation starter. If you are worried about rust, then a can of clear coat that is UV resistant.