If you ask Weshty here (SSM) nicely, he'll fold the sides for you on a rolling machine beforehand and enniscorthyman is right about scoring the sides etc. As for primer, it's a must as paint and brass don't like each other. I dunk the brass parts in white vinegar first for an hour, wash with wire wool, and then use a coat of airbrushed varnish as a primer, and it works fine, but it needs a day to dry.
Halfords sell metal primers which I believe are equally effective. R