This is Sally Hansen's Hard as Nails Clear Nail Polish #800 (UPC 0 74170 38283 9), which is a good, easy way to arrest clearcoat clipping or flaking, or art chipping on the playfield before you apply mylar.
This is available at stores, but some areas of the US don't seem to have it readily available, so I'm making it available here as a place of last resort to get it, since it's a hassle to ship, cannot ship international or domestic by air, and requires a hazardous sticker on the package. If you can find it locally (do NOT get the "strengtheners" "hardeners" etc, only the clear nail polish, color #800), it will be cheaper, but if you can't, you can at least get it here.
NOTE: I CANNOT SHIP THIS INTERNATIONALLY, ONLY WITHIN THE CONTINENTAL US VIA GROUND due to its flammable profile (no Canada, or Hawaii, Alaska, Puerto Rico, etc since they require air shipping).