You have to go back to the late 40's early 50's to find a PF that didnt have SOME sort of protective coating on it.

ALL have yellowed over the years and are what gives many machines that "old" warm look. But in all honesty, these games had BRIGHT whites, yellows, reds etc .... remember... they had to catch your eye when you walked into an establishment and make you want to play them. These machines were VERY bright indeed!

Mylar was really used at factory for high traffic areas and was laid down to assist operators to make the games last longer. Ops didnt have time to keep going round waxing PFs... that kind of maintenance went ignored.

Waxing is all thats required for home use or any use for that matter - we can do this as we have the time and inclination.
You are though waxing over an already protected PF, be it linseed finish, Diamond Plate or CC... the era of the machine determined what product was used not whether it was used or not