Tried a rotary brush on a nasty flue last Tuesday and it beat my old flat wire brush by a mile
Had a job in Springfield with about 12 years of creosote baked onto a clay liner, and I ran my usual flat wire brush first and it just polished the surface. Switched to a rotary brush on the same drill and it actually broke the crust loose in a fraction of the time. The difference was the spinning action getting into the grooves instead of skipping over them. Curious if anyone runs rotary on every job or just saves it for the rough ones.