I use to on all my balls and it was great but I have since changed span and pitch in the thumb itself and haven't yet got one that feels good enough to mold.  It was great though I had one molded before with two pieces of tape cause i like the feel of tape but it was the right size my thumb never got bigger than the mold and if it got smaller i could just add tape as needed.  It really helped ball to ball cause if I lay a ball down with two pieces of tape in it then the next ball i pick up I know needs two pieces of tape in it also.  Now since i haven't yet molded a new thumb one of my balls has 5 pieces of tape then the next ball 2 then 3 and so on it really did make it alot easier.
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I should just quit bowling, oh wait I already tried that.
Now that I am back and my Saws are sharp again, I am ready to cut some wood.