You can test it on a spot, avoid the decals to begin with. It might not improve the finish to be to be worth it, but it should leave a protective film. Shellac is actually very hard an durable, I guess any damage often comes from damp storage or more mechanically. The line seed oil method described in this forum does a good job. It is a bit of work, but you can put it off until later when you are up to this kind of job, and still use the machine.