There is no good DIY coating for tubs.
Hire a company that all they do. There going to acid etch it, recoat it with gel coat.
It will then last at least 10 more years.
A DIY paint job with one of those coating kits sold in the store will only last a year or two even if it's done perfectly. And when if fails someones stuck having to get all the old stuff off before it gets redone right.