Padudle,
In my experience, the Sherwood teflon HP seat is pretty tough and lasts for hundreds of dives. Then it can be reversed and used for hundreds more.
Some years ago on a caribbean island I heard some of the divemasters talking about a "Sherwood tool". When I asked, they showed me the famous item. It is a straighened paper clip held in a pair of pliers, and is used to pop out the HP seat. They use a sewing needle for the LP seat. They also said that when they had used both sides of the seats, and no new ones were available, they rubbed the seat on fine abrasive cloth to remove the groove. I have never used the abrasive cloth, but the Sherwood tool does work.