Bathtub deliming products like C.L.R. or LIME OUT improve the finish of dull tubs. These bathtub delimers are less hazardous than some others.
If you have been down the C.L.R. and LIME OUT road without any success, you can go for the big gun, Rooto Toilet Delimer. People capable of working with strong chemicals should only use it because you will need gloves, eye protection and proper ventilation. It is so strong it will dissolve years of built up lime in old toilets. Extra care must be taken when using it on an open surface.
To use the Rooto Delimer, get a long handled brush with soft plastic bristles and a plastic bucket. Pour some of the delimer into the bucket and use the brush to apply it to a small area on the tub. Let it sit a minute or so, apply a little more then rinse with clean cold water.
If it worked you now have a procedure that will save that old worn out tub. You may just get a few more years before you have to replace or reglaze.