I think your best bet would be to buy a new tub OR tell the new owner that the tub needs to be replaced and work that into the cost of the house. Let him try refinishing at his own cost should he want to try that.
You see, there is a very big difference between a powder coating or porcelain enamel and a "paint" (or any other field applied coating like paint). Your existing steel tub isn't painted; it's powder coated, and that involves a completely different technology than traditional coating methods like painting (with epoxy paint or whatever else). Powder coating is a technology that was born in Australia after WWII, and has since spread to the rest of the world.
Powder coating involves electrostatically spraying a mixture of very hard plastic (usually polyester) resins and coloured pigments onto the surface of a metal part. Because the plastic resins and pigments have one electrical charge, and the metal part has the opposite electrical charge, the resins and pigment particles are most strongly attracted to the surface of the metal part where the accumulated powder is the thinnest (and so the electrical attraction the strongest) and that promotes a uniform coating thickness over the entire surface of the metal.
The coated metal part is then baked in an oven at (typically) 350 deg. F for half an hour (just like cookies) and during that time the hard plastic resins melt and flow together, thereby coating the surface of the metal with molten plastic. The coloured pigments are encased in this plastic very much like the raisins in raisin bread, giving the otherwise transluscent plastic it's colour and opacity.
When the metal part is taken out of the oven and cools, the powder coating process is complete, and the coating will be several to many times as durable as field applied coatings like epoxy paints or moisture cure or catalyzed waterborne polyurethanes.
The process of electrostatically spraying a plastic resin on a surface and then baking it to melt the plastic is practiced from baking temperatures from as low as 250 deg. F all the way up to temperatures of over 2000 deg. F. Coatings that are baked at temperatures lower than 700 deg. F are generally called "powder coatings", while coatings baked at temperatures above 700 deg. F. are generally called "porcelain enamels". The process is exactly the same, but the harder coatings baked at higher temperatures find applications in different industries than the less expensive coatings baked at lower temperatures. Nowadays, some companies are even powder coating WOOD parts by dipping them in a vat of liquid (which presumable contains the plastic resins and pigments) and then baking to melt or cure the plastic. This powder coating of wood is new technology, and it's something I know little about, other than it exists.
You simply cannot match the durability that can be achieved with powder coatings with the current technology of field applied coatings, and that's what people don't seem to understand when it comes to having their bathtubs refinished. You simply can't heat up the bathtub to 800 degrees F. without burning the house down, so bathtub refinishing has to be limited to field applied coatings. Those field applied coatings might look great when it's new, but it simply isn't as strong, hard and durable as the original powder coating that tub came with, and it simply won't stand up nearly as well for nearly as long as the original powder coating did. Until coatings technology changes, refinishing a tub is a short term cosmetic improvement, not a repair.
My understanding is that bathtubs have gotten cheaper since the 1950's and 60's when they used thicker steel and a thicker powder coating. But, I'm told that American Standard's "Salem" line of steel tubs uses a heavier gauge of steel and a thicker powder coating than you typically find on tubs nowadays. If it were me, I would check out the offerings of the higher end plumbing fixtures like Kohler to see if they make a truly durable enamel steel tub like American Standard and Crane did decades ago.
With powder coatings, generally the higher the baking temperature, the harder and more durable the coating. The hardest powder coating in your house is probably the blue/grey "ceramic coating" on the inside of your stove's oven that was baked on at approximately 1300 deg. F. The hardest coating in your house, bar none, is probably anything that's nickel plated. Typically, nickel plating is done prior to chrome plating (they plate chrome over the nickel) and my understanding is that nickel is harder than chrome (but not as pretty).
(PS: the ceramic coating in ovens in modern self cleaning stoves is exactly the same as that in non-self cleaning stoves. Back in the 1970's, some companies sold stoves with a catalyst applied to the inside of the oven to help burn off grease spatters. Those ovens were called "Continuous Clean" ovens, and the catalyst was damaged by strong alkalies like oven cleaner. To this day, some people will still tell you that oven cleaner will harm the ceramic coating inside a modern self cleaning oven. This is completely false. I own a small apartment block and I wouldn't have purchased 19 brand new self cleaning stoves for it if the tenants could inadvertantly wreck the ovens by using oven cleaner on them. To this day, one of the reasons why landlords buy NON-self cleaning stoves for their rental properties is because they've been told that a tenant can damage the self cleaning oven by using oven cleaner in it. People make some of their most important decisions based on bogus information, and it's tragic, really.)
Anyhow, until they can come up with a field applied coating that can match the hardness, strength and durability of the powder coating on a new tub, the bathtub refinishing business is going to remain in the same bag as the used car sales business. Promise much, deliver little, and be sure to operate as a corporation so that the company's director and sole shareholder can't be held personally liable for the company's legal obligations.
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