I use to do that a lot. One build had the Glyptol come off in sheets. I'm very particular about the metal prep before painting the inside. There is such a thing as oil impregnating the iron. Found this on other industrial iron that I had painted in the past. I now remove all the foundry flashing and prep whatever I can to make the cast iron as smooth as possible. Prep time vs the price of Glyptol is worth it.