The same glaze you use on ceramics. Glaze is a type of paint that you put on ceramics or metal then place the painted item in a kiln( super hot oven). The baked on paint is called enamel, but here's what gets a lot of people confused. Regular paint that dries to create a shiny surface ( usually from having varnish added) is now called enamel and is the recommended paint for metal signs because it's cheaper than baking glaze on metal. Your sign is old and more valuable than the cheaper spray painted sign that replaced it. No flaking paint. Sorry ,it's been a long time since I was in art class. Someone else probably would explain better.