If you want to fix a chipped bathtub on your own, a two-component epoxy kit with a catalyst and a hardener is your best bet.
Start by scrubbing the spot with something like dish soap, detergent, degreaser, or Bar Keepers Friend, which has a bit of grit
Then, wipe the area with rubbing alcohol or acetone to get rid of any residue that is still there.
Mix the epoxy compounds by following the directions on your repair kit.First, mark off the area you're going to fix with painter's tape.
A layer of epoxy usually needs between three and five hours to dry, but if you can, leave it for 24 hours to be sure it's set.
Use ultra-fine sandpaper to make the edges of the repair blend in with the rest of the tub