Is there a trick to clean bathtub?

What's it made of? Older places often have enamel or cast iron, and that takes a bit more scrubbing than acrylic. Either way, baking soda paste with a little dish soap on a non-scratch sponge does most of it. For the grey limescale line, lay paper towel soaked in white vinegar over it for an hour. Just never use vinegar and bleach in the same session.