You can etch the brass, but it isn't necessary. Clean it with soap and water, let dry, and prime it. Let dry 72 hours, then apply paint. Let dry 24 hours, and apply a varnish.
Either enamels or acrylics will work just fine. Modern acrylics are every bit as good as enamels -- better, in my opinion, because clean-up is easier and there is no health hazard. I use Vallejo Acrylics exclusively now.
Connecticut Valley Railroad -- A Branch of the New York, New Haven, And Hartford
"We don't stop playing because we grow old. We grow old because we stop playing." --George Bernard Shaw