I use Daryl's method and prefer it to using a hard substitute for the thumb. The thumb, being somewhat less rigid, catches the tips of each side of the lands and gently and evenly removes that sharp corner. It's those corners of the lands that is stretching the patch material past it's ability to keep from cutting. If the patch material is jeopardized going into the bore, how can it be expected to withstand that trip back out of the bore under unfathomable pressure and speed. (UNFATHOMABLE: unable for me to get my head around, yet able to be measured scientifically)