Osvaldo, this is a job for the professional tub painters because of the VERY dangerous chemicals that are needed to prepare the tub for painting. The process starts with sand blasting so the type of tub will dictate whether you want this done in your house or to send the tub out for the processing. Another option is to have a fiberglass liner installed.
Any "paint" that stands a chance of sticking for more than a few months will be one that is a two part epoxy. These may last for a couple of years and are capable of being sprayed with an high pressure paint sprayer. They are not for brush painting or spray painting by someone not familiar with the techniques and equipment necessary for using this kind of paint. There are some paints available that will do even better but these are also so dangerous to use that most pros will not use them at all and definately not in a residential area.