I'm thinking tar and bug remover is ok if it is clear varnish or lacquer on the machine - but you are likely removing shellac under that black stuff. I had one not long ago I used tar and bug remover on then had to get some shellac back on the machine. One way you can kind of tell if you are getting too deep is to watch to see if the black stuff is turning brown. If so you went too deep. Glenn should be back in a couple days or a week and he will have some ideas about this.