Scratches caused from kid's toys.
Scratches caused from shower attachment hose.
Or scratches that just 'appear'.
Don't worry. You have everything you need already available. You just need some toothpaste squeezed directly onto the bath enamel and a wet cloth pressed around your finger.
Oh, I forgot, you need a small amount of elbow grease.
We use toothpaste on our teeth everyday. I'm sure it is safe to use monthly on a bath. Disclaimer: I can't be held responsible for any damages this may cause your bath. Please test small areas first.