I swore by the stiffest poly brush for years, then a 3 inch wire brush on a drill saved me on a tarred-up flue in Akron
Took a job on a 1920s house where the creosote was baked on like glass and my regular brush just polished it, so I tried the drill brush at low speed with a shop vac running and it flaked off in sheets, but I'm still not sure if I got lucky with that particular glaze, so has anyone else had it work on the hard shiny stuff or did I just get one good flue?