If the shellac is compromised, TR3 will remove it. It happened to my 201-2, but only on the top of the arm, not on the bed. I just went with it and let the bad stuff be removed. The TR3 is a protective product though, and I've not had any more shellac come off other than where it was bad to begin with. I've not had it damage any decals on the machines where I've used it, but all were still protected by the shellac. The biggest advice I can give is to start in a small area and follow the directions on the can. Don't let it dry until you're satisfied the machine is clean enough, and change your cotton ball often. Once no more dirt/grunge/whatever comes off on the cotton ball, then you let it dry and buff it off.
Cari