So my question is, doesn't anyone refinish a tub with old fashioned porcelain? Is this just an obsolete process now?
No, it's not at all obsolete, and new powder coatings are being developed all the time. In fact, you can even get teflon coatings baked on at most shops.
Really, what the problem here is that you don't understand the powder coating process well enough to realize that it simply CANNOT be done in a person's house. Lemme explain it:
The porcelain enamel on your old bathtub is not applied like paint. Porcelain enamel is applied by the same process as "powder coating". In this process, a mixture of synthetic plastic particles and coloured particles (called "pigments") are electrostatically sprayed onto a metal part. That is, the metal part is charged positively and the gun spraying out that mixture of particles is negatively charged. Thus, the particles coming out of that gun are also negatively charged so that they coat the positively charged metal part fairly uniformly. That is, they are attracted more strongly to the more positively charged areas of the metal part where the coating is thinnest, so this process tends to coat the metal uniformly with powder.
That coated metal part is then baked in an oven for anywhere from half an hour to 4 hours at 350 to 1200 deg. F. During this time, the synthetic plastic particles melt and flow together to form a continuous film over the metal part with the coloured particles suspended inside it much like raisins in raisin bread. But, just as importantly, the plastic used in the resin particles "cure" at the elevated baking temperature. That means that chemical bonds form within and between the plastic resin particles to make the plastic heavily "crosslinked" with chemical bonds, and that results in the coating being much harder, stronger and resistant to heat when it cools.
When the metal part is taken out of that oven it is said to be "powder coated", but that's a misleading description to anyone not aware of the process. The coating was only a powder prior to baking. Now it's a solid film that's very much harder and more durable than air dry coatings like epoxy paints, two part acrylic polyurethanes, moisture cure polyurethanes, etc. And, therein lies the problem. These field applied coatings simply can't match the hardness and durability of powder coatings, and you need that high hardness and durability to provide a 50+ year lifespan on a bathtub.
Steel and cast iron tubs come powder coated from the factory. But, to recoat them you need to sand blast the old powder coating off and powder coat them again. This could probably be done by any powder coating shop in your town, just look under "Industrial Coatings" in your yellow pages phone directory. However, the cost associated with removing the old tub, sand blasting it, recoating and baking it, and then reinstalling it would be more than the cost of installing a new steel or iron bath tub.
So, no one offers an in-house powder coating service because of the impracticality in doing it. How would they heat up your cast iron tub to 400 degrees without burning your house down? THIS is why you've come up empty in finding someone to powder coat your bathtub.
You don't need to know the rest:
Generally, if the baking temperature is less than 500 degrees F, the coating is simply referred to as a "powder coating". However, if the baking temperature is above about 800 degrees F, then the resulting coating is generally called "porcelain enamel". That's cuz the coating itself resembles more of a ceramic material. Porcelain enamels are also often referred to as "ceramic coatings". Generally, the higher the required baking temperature, the harder and more durable the coating will be.
There are several examples of powder coatings in your house. For example, the side panels on your stove are only painted, but the cooktop itself and probably the front panel on the oven door will be powder coated. That's cuz these parts will get much hotter than the side panels when you're baking something, or self cleaning the oven, and powder coatings stand up to high temperatures much better than field applied coatings do. Field applied paints would soften up when they get warm, and sliding a pot across that warm cooktop would scratch up the paint.
The interior surface of your hot water heater's tank is powder coated to protect the steel walls of the tank from corroding.
And, probably the hardest and most durable powder coating in your house is the blueish gray glass-like coating on your stove's oven walls. That coating was baked on at about 1200 deg. F.
You can learn more than you need or want to know about powder coatings and porcelain enamels at:http://www.powdercoating.org/home.htmhttp://www.porcelainenamel.com