We are painting steel door frames. They were first primed and painted with an enamel paint, which has been very durable. Someone thought that they were helping by painting over the enamel with a latex paint, that is now flaking off. What is your suggestion for preping before we paint again? We have scraped and sanded. Some areas comes off real easy and some are like cement. We have purchased a latex enamel to repaint. Thank you.