The “go to” sealer for flaky backglass on old vintage pinball machines is Krylon Triple Thick.
Stop further damage from flaking paint cause by the hot lights in the pinball backbox. Repair and repaint what you can and then seal it with Triple Thick.
Krylon I00500A00 12-Ounce Triple Thick Clear Glaze Aerosol Spray,High-Gloss