These days almost all new baths are made of plastic. However, older enamel baths are still around because they are very hard wearing and last many lifetimes. Usually with a good clean a tub that might be 100 years old will still come up looking as good as it did the day it was installed.
However, when an enamel bath is chipped it rapidly looses its looks and is no longer a luxurious place to sit and get clean.
Firstly fill the chip with automotive bodywork filler. Use the type that
comes in two tubes (a filler and seperate hardener). Mix as per the manufacturers
instructions and spread a little over the cracks. Leave the filler a little
bit proud of the bath surface and then once it is set hard (leave it overnight),
sand it back down to a level surface with damp, fine grade 'wet and dry' sandpaper.
Now we need to apply enamel paint (like very thick gloss) over the filled area. Once the paint has dried, sand again with wet and dry to bring your bath back to a smooth surface.
Finally, a body restorer polish from the car supplies store (halfords for example) is perfect for bringing the bath back to a high gloss finish and make your chip totally invisible.
Apply liberally to the bath, wait an hour and then buff furously with a lambswool buffer. Now repeat this last polishing step 3 or 4 times to give yourself a very smooth, shiny bath.
If your bath has a large area that needs repairing, consider sanding back to bare metal and resurfacing the entire bath. This is not actually as big and scary a job as it sounds. There are a number of enamel bath restoration kits that contain eveything you need for doing this as a DIY task.
Your bath will be out of action for a few days, but it really isn't too difficult.