flex thru damper area, too thin?

[woodsie8]

I had a installer tell me that some flex liners that are used for the damper area, are so thin that he has seen the brush go thru them when you clean the chimney. He said you can cut it with a pocket knife. He is too far away to come and do a bid but told me to make sure I ask for heavier flex for the damper portion.

 

[STOVEGUY11]

They make poly brushes for metal chimneys, and I hope no one is trying to cut your liner with a knife after it is installed.