Quoted from jrpinball:
Why would alcohol affect the clear and not the colored layers below? If what you're saying is true, the clear is what's holding much of the dirt, being that it's the upper layer. If it's dirty, and likely compromised to begin with (I don't think it's very thick), it's probably better to remove it, and protect the remaining artwork with several coats of wax and a new ball. For home use, this should preserve the playing surface indefinitely, as long as it's waxed periodically.
I am not a chemist and can’t offer an explanation as to the why. All I can tell you is what other experienced pinheads have reported coupled with personal experience. For a relatively slow moving em, you might get away with stripping off the clear (likely lacquer) for awhile if you’re REALLY meticulous about cleaning and waxing regularly, but eventually it may catch up with you.
Case in point: I took the alcohol approach with my first pin years ago before I knew what I know now. I cleaned a really dirty Space Shuttle with 91% isopropyl alcohol. It cleaned up really well, but also dulled it slightly. I was meticulous over the years about regular cleaning, waxing and using only new balls which I would change fairly frequently. For the most part, the playfield survived in its post-cleaning condition with the notable exception of high repetition ball travel areas. After roughly 4 years of home use, the paint in the pop bumper area, rollover lane exits and targets had worn to wood. Similar games which I have cleaned only with naphtha have not suffered this type of accelerated wear.
I do think that so long as nobody has stripped the original clear that you have to be careful about choosing between cleanliness that is prone to potentially accelerated wear or a dirtier looking playfield that will survive longer due to the additional sacrificial layer of dirty, thin, aged clear.