Had to pick between a poly brush and a steel wire one for a 1930s clay liner job in Portland last week. Went with steel because I figured it'd be tougher on the creosote. Big mistake - it scratched up the liner bad and the homeowner noticed right away. Anyone else had a poly brush hold up better than you expected?
Sand embedded in polyester bristles is a TERRIBLE idea. Those grains will just act like loose grit and gouge the liner over time. Poly brushes need to be CLEAN and rely on stiffness, not embedded abrasives. Try a rotary chain knocker for heavy creosote instead of making a homemade scratch factory.
Hold up on the sand idea. Mixing sand into a poly brush sounds like a quick way to turn a decent tool into a scratch monster. Sand grains are basically tiny rocks, they'll do the same damage a wire brush does, maybe worse because they can get embedded in the bristles. Poly brushes work best when they're clean and just rely on the stiffness of the fibers to knock off loose soot. If you're dealing with heavy creosote, try a rotary chain knocker or a chemical treatment first before scrubbing the liner with anything abrasive. Did you check how thick the glazing was in that 1930s liner before you picked the brush?