Quoted from arqpuebla:
Isopropyl alcohol is not recommended for cleaning 80s pf, because is too abrasive, clear in that era of PF was too thin and/or with less resistance vs 90s pf's clear coats.
So, it easly eat the clear on 80s pfs. getting the artwork more propense to be damaged in the process of cleaning.
In my experience the thin layer of original clear contains dirt and is yellowing. For me that are two reasons to remove it. As the playfield will be clearcoated anyway there is no point in trying to keep the original clear. I use nitro thinner to remove it, applied on a cloth. Gently rubbing to remove the yellowed layer of varnish, it is more efficient than magic eraser and safer than sandpaper.
As an example: it took me about 10 seconds to clean up the "10" below, removing the old clear and ball swirl dirt.