Guy at the supply house in Tulsa said my brush was too big for clay flues, and he was right
I been sweeping with the same 8 inch poly brush for years on everything, figured one size fits most. This older guy at the supply house in Tulsa watched me load up and just said "you're gonna scratch the hell out of them clay liners with that thing." I brushed it off for two jobs, then the third homeowner called me back because she could see marks in the flue. Switched to a 6 inch soft poly for clay and my callback rate dropped to almost nothing. Anybody else keep a separate setup just for older clay flues or am I late to this?
Poly brushes come in different stiffness ratings and most guys never bother checking the label before buying. A stiff 8 inch will chew up soft clay, but a soft 8 inch can work fine in a lot of flues if you let the brush do the work instead of forcing it. Sounds less like the size was the problem and more like the bristles were too stiff for what you were running.