What is the question; flow, adhesion, making it wet, controlling the flow so it doesn't overrun, something else?
This should be pretty easy depending on the geometry.
Basically the solder will flow on nice clean areas. If you have a sharp edge and you control the heat properly the solder will stop at the edge. If you want the solder to stop some place you can use a braze stop. Maybe just etch it with acid, use oil or grease, use milk of magnesia or buy a commercial braze stop which I think is sold under the name NICO braze.
