Man oh man, @richardh44 you hit the nail on the head there. A buddy of mine spent a whole weekend scraping out an old frame only to find the channel had a hairline crack he missed, then had to swap the whole thing anyway. He said he would have saved himself the headache if he'd just checked the metal first like you said.

That inch of glazed creosote you pulled off, was it from a wood stove or an old fireplace insert? The reason I ask is that I've seen those 9-inch poly brushes with steel bristles really shine on the heavy stuff, but they can scratch the hell out of a stainless liner if you're not careful. I had a buddy who swore by his dad's setup too, but he gouged a cheap liner so bad on a job he had to replace it for free. So I wonder if that old school performance only works because the flues back then were built tougher, or if we're just babying our gear too much now. Maybe it's just that the new liners are more delicate and the brush needs to match them, not just the soot.