Put the shine back in your porcelain sinks and bathtubs by giving them a good scrubbing with full-strength white vinegar, followed by a rinse of clean cold water. To remove hard-water stains from your tub, pour in 3 cups white vinegar under running hot tap water.
If the shine is gone due to harsh chemicals or age and not simply cleaning, you made need to refinish it.
Bill,. Start by cleaning the tub real good, let it dry completely .Close the drain, and use white vinegar in the tub. Swish it all around, and up the sides with a sponge. Don't rinse, just open the drain. Aloha!
Bon Ami Cleanser applied to damp tub surface, scrubbed following surface grain of porcelain will return the shine, unless finish has been damaged.