Chips and cracks in your ceramic or porcelain bathtub are no cause for worry. As long as the crack has not run through the entire base of the bathtub from one end to the other or turned into multiple cracks, this project can be a simple repair. There is no need to call the professionals in on this problem, because you can do it yourself.
Fill the tub with hot water and mild detergent. The suds will clean off the tub walls to remove soap scum and other grime. Drain the tub. Scrub tub with a sponge and rinse. Dry completely.
Sand down the surface surrounding the damage. Gently move the sander in a circular motion for a few seconds to roughen the surface without removing layers of the ceramic base.
Mix the filler and hardener in the bowl. Stir it with the putty knife.
Use the toothpick and applicator to carefully fill the cracks. You can overfill the cracks to make sure they are repaired properly. Dampen a towel. Carefully wipe away the excess from around the repair..
Let the filler dry for about 24 hours. Spray on the acrylic enamel to apply a topcoat and seal the repair.