Protecting area around pop bumpers is absolutely necessary. This is usually
the most worn area on any game. Many machines were protected around the
pop bumpers, starting at the factory, with a circle of self-adhesive mylar.
If your machine doesn't have this mylar you need to apply some.
Obtain some self-adhesive mylar at a plastics store and cut a circle using a
tuna can as a guide. Apply the circle to the playfield when you have the
bumper completely removed and use an exacto knife or razor to trim the holes
in the playfield for the bumper components.
I recommend not using the non-adhesive mylar. Gottlieb machines come with
non-adhesive mylar. If you could keep the mylar always perfectly clean this
would not be a problem. But there is always some dirt floating around in
games. Every game I've seen with self-adhesive mylar around the bumpers has
had the area protected. Every one I've seen with the non-stick mylar had
the paint underneath worn to chalk.
It's not hard to pull off the self-adhesive mylar and replace it, and
chances are you'll have to pull the bumper off completely anyway more often
to replace a broken skirt or bumper body.