There's a LOT of commercially available patina solutions for aging metals. Usually a nitrate or liver of sulpher based for brass. Look in your local fine art or even hardware store paint sections - where faux finish supplies are. Or just google.. Remove the laquer finsh first, if it's coated, then apply the solution as directed..
If you ever want a temporary patina, then ultra-matte finish "railroad" model colors work well stipple on or apply with an airbrush....Do your research on the type and color of patina that you want first..Look at pics of the real aged thing.
Jayn