Not defending Stern and this can actually be a criticism, but when talking about old EM's or a Flash etc and probably most B/W games I do not recall such games having many if any Airballs. Now that probably is because they were well designed and in the case of EM's things are moving much slower. So if a ball is bouncing around the playfield a lot, multiballs a plenty all contribute to dimpling.
So this could be one explanation or contributing factor on top of softer pf's. What caused divots around ramp ball drop areas on B/W games. I'm guessing thousands of balls dropping in the one spot. My thoughts anyway
EMs and SS machines up to the early 80's didn't have a clearcoat. That's another hole in Steve's argument ("60 years in pinball"), because if you knew the facts up to 60 years ago, you would know that manufacturers used varnish. They also used STRONGER TIMBER. In fact, it is only recently that this has become an argument - because Stern use weaker timber and a clearcoat process that is over 15 years old.
The timber is softer, and the clearcoat is old technology.
Thats why when we apply modern clears to older playfields - guess what - They don't dimple anywhere near what new Sterns do..