Mix together fiberglass epoxy with the resin included in the kit, then fill any cracks or large chips with fiberglass epoxy using a small trowel. Allow the filler to dry and cure for 48 hours.
Sand the entire surface of the bathtub using an electric sander with a medium-grit sanding belt. This will remove the old glazing and smooth down any areas of roughness left behind from the epoxy patching.
Vacuum the bathtub with a shop vacuum, then wipe it down with a damp washcloth to remove dust and debris.
Fill a paint tray with porcelain bathtub glaze, then coat a 3-inch round paintbrush in the porcelain bathtub glaze.
Apply the porcelain bathtub glaze over the entire tub in an even layer, then allow the tub to air-dry for 18 to 24 hours.
Paint a second layer of porcelain bathtub glaze over the surface of the bathtub, then allow the second coating to dry for another 18 to 24 hours before use.