I haven't used BB but if I were, I'd probably save what I shot the frame with and use it to do the sheet metal. It will be smaller and less aggressive.
I believe an 80-120 grit would do what you want.
Remember to back off the pressure to 45-55 psi for the sheet metal and keep the angle of the 'jet' around 45* to minimize any warping.
Others here may advocate for silica play sand... but I'd rather use a safe blast media.
