Bushings are considered "Consumables" meaning that they are used like sandpaper and should be replaced when they are used up . If you are using regular stock bushings that means after they are no longer the right size you just replace them with new . Most bushings only cost $3 to $5 a set . If you are using special bushings , like JohhnyCNC's between center bushings then more care is needed due to the price of replacements .
A more important reason you should be careful and not sand the bushings is because the sanding dust from the bushings will show up in your finished pen as little specs and stains in your finish .
While it doesn't address the staining issue, if you use high quality between center bushings for turning, you could always use standard bushings for sanding. This way you are only wearing down cheap bushings at a point in the process where the high quality bushings are no longer critical.
My friend JohnnyCNC sells these and that will cure your problem:http://penturnersproducts.com/index...id=166&zenid=1b77d3a0035c29bb5182e7dfa536297b