Can't help wondering if a lot of these issues are caused by change in temperature and humidity exposure between clear coats.
I'm no painter. but I imagine temperature and humidity fluctutation's would play a big part.
1.How long the playfield sits after the last clear coat before the game starts assembly. You would like to think at least 2 weeks but who knows?
2. Are the Playfields supported fully during and after clear coating (.IE not sitting on 2 trestles)?
3. Are they under a temperature & humidity controlled environment?
You can afford to cut corners on just about everything on a Pinball Machine during manufacture EXCEPT the Playfield. I can't believe Stern would be silly enough to skimp on the only thing you can't afford to. Can you imagine how much it would cost them on a warranty claim to replace a playfield....OMFG.