Epoxy paints are available that you can use if you want to do this job yourself, and they are quite durable. However it is very difficult to apply them without seeing the brush marks.
The real professional method, short of taking the tub out and sending it out for re-finishing, is to have a specialist come in with catalytic spray finishes (usually highly toxic).
He or she prepares the surface, ventilates the room to the outdoors, wears full protective gear, and sprays on the special paint. This works very well.
This job was beind done by Worldwide Refinishing Systems, part of the DreamMaker franchise.