Only use a dremel or other roto tool if you are sure you can control the thing. I used to carve Magnesium speed plane parts w a dremel because I was in HS and did not have the right tools. The amount of chatter you will get with anything other than a disposable abrasive drum is scary and will ruin your roto tool's bearings very quickly. Assuming the dropout is alum, the abrasive head will clog very quickly and you'll have to get another, and another, so don't bother w the stone stuff. If it is steel, the abrasive will last longer. Look out for heat build up. Not sure if they make a sanding drum that is small enought in dia for the dropout. If I had to do it, I will use a good rat tail file, and just burn the elbow grease.