steel putty to repair rusted through tub drain
I had a similar problem in my steel tub. The tub itself around the drain was rusted all the way through. I took out the drain, broke off most of the really bad rust, and got some moldable steel putty. (Not sure of the name, but it came like a fat crayon, bought it at Ace hardware). It says you can use this stuff to repair engine parts! I formed this underneath the tub, pressing it tight against the existing opening and shaping it. After a few hours, it was solid as steel, and I just painted over it with some of that porcelain touchup paint and it looks good as new. I put back in a new drain with fresh rubber gasket and it works great. It's been a little over a year and still holding up, but I know there's still rust under there, as I can see a little bit of rust stain coming through the porcelain paint, but for now it's still strong. Had I known how good this steel putty was, I'd have done a better job removing the rust beforehand.