You need a methylene chloride stripper to soften and remove 2-part cured epoxy. The porcelein finish should not be affected, but may have been etched in preparation for the glazing treatment. You should not expect that the tub is in good condition under the glaze, and may need to be professionally refinished. Check out Peel Away2 at peelaway.com or find a methylene chloride paste stripper at your store. Abide by the warnings concerning fumes and ventilation.