Try using a soft bristled brush with a vinegar and baking soda paste to remove whatever is on it and then hose down with hot water.
If the finish has dulled over time nothing can bring it back without refinishing.
Clean the tub really well and then add hot water and vinegar. Let sit for a few hours and then wipe clean with a sponge. I read that you shouldn't let it air dry or else it could leave water stains. Good luck!
You don't say what your Bath is made from. If it is Soap scum build up and it is a Cast Iron Bath, Clean it with something like Vim or Bar keeper Friend If it is Acrylic use,Non Scratch Sponge and w/up liquid.