Why,unless it's a Norge,Admiral,Signature,Bradford,JennAire,or Wizard all of which had painted tubs,would anyone want to paint a porcelain or stainless steel tub??? I run a gallon of muratic acid in a full tub of hot water and it eats up all of the rust and any build up of calcium,lime or zink.
I had a Frigidaire wash/spin tub sandblasted and painted it with POR-15, in silver of course, and it turned out beautiful. It was a dreadful rusty mess from the South Dakota Farm (all the Frigidaire tubs were destroyed from the elements out there) so I drilled out the balance ring and removed the metal shavings and then had it sandblasted. I filled the balance ring with cement mix for ballast after it was done.
For your drain-tub, Brian, it looks like you have a good start. You can fill the holes with a little JB-Weld or silicone as long as you cover all areas of the hole so no moisture can get to the rusty areas and continue it's destructive evil. POR-15 is made to paint over rust (P.O.R.) so it will not adhere to anything with gloss such as porcelain, etc. It should be bare (or primed) metal or rusty metal for it to work properly. I've touched up a few rusty spots on a porcelain outer tub before and had tiny bits of paint in the drain-water for months afterward. The only place it stuck was onto the tiny rusty spots, the rest just washes away.
Here is the '59 Frigidaire spin-tub - ain't she pretty? ;-)
There is/was a company in IL or IN that did re-porcelain coating for stoves, etc. and it can be done but it is very expensive. Check with the stove restoration websites as I know they use that company for burner jets, grates, panels, etc.