Tumblers do work well. The vibrating tumbler does not take off as much material and is faster. However, a tumbler will not work well on bumpy surfaces. But considering the acid resistance of boro this is one of two choices.
http://www.therockshed.com/tumbler3.html
A nice SC grit perhaps 200 to 400 would work well. Since there is no desire to polish the bead a one shot approach is all that is needed. i.e. to frost the opposite approach is needed.
