Holy Bathtub Batman!

A few years back I noticed a hole had developed next to the drain in our 50 year old porcelain on cast iron (?) bathtub. At first I sealed it with RTV and painted it to match but then I got concerned that this was an inadequate seal.
What I've been doing is to cover the entire drain area with aluminum tape, the kind used to seal ducts. I remove the grill, run several well- overlapped strips, puncture the opening and seal the sections down and replace the grill. Using a CD as a guide I cut the tape into a nice tidy silvery circle. For a kludge it actually looks pretty good. I say this like it's an ongoing thing because it is. The tape oxidizes so I redo it every so often.
This is a smaller lav and the tub is boxed in at both ends and the side. And above by ceramic tile on all 3 sides. I'm not sure how it would be removed for replacement; maybe they'd have to cut a wall. The issue hasn't been aggravating enough to go down that path just yet.
I know the obvious solution is to call one of those firms that covers an existing tub with a plastic liner. Sorry but no thanks. Way too much like a cheap motel. I'd rather keep my aluminum tape circle. Maybe if we sell the house we'd have that done but not for my own use.
So a couple of questions:
Is there professional solution aside from full replacement or the plastic liner? What would they do?
Secondly, I'd like to revisit sealing this thing, maybe with epoxy or something like that. There is of course a space under the hole so I'm thinking maybe something needs to be shot in there to fill in. Probably not gallons and gallons of epoxy though. What would work? My first thought was to shoot in as many tubes of cheap latex caulk until the pile builds up to be close and then epoxy the rest of the way to close the opening. But maybe there's something better like vinyl cement to shoot in there.
It's not possible to get to the space underneath from behind. If it were possible to make a small opening in the wall behind the tub I'd wedge some wood down there to get up close to the hole. Alas, beyond the wall are kitchen cabinets with countertop and ceramic tile above that. So pulling the cabinets to get to the wall is too huge a project for what is still a kludge repair.
I don't mind the aluminum tape thing and might still do it as further assurance of sealing even with an epoxy repair. I'd just feel better if the hole itself under the tape was closed.
What's the best way to close the hole and, assuming something needs to fill in underneath first (maybe not?) what's best for that?
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The best solution is to have a stainless steel ring machined in a lathe. Make it the proper dimension and you should be able to permanently glue it in place.
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wrote:

You can also buy stainless tape.
I would try Plumbers Goop instead of rtv. Needs something to hold it because its runny. Clean well after removing rtv silicone. You can paint over the goop.
greg
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So what would you fill in underneath? I'm back to my idea of shooting in as much cheap latex caulk as it takes to build up a pile within a reasonable distance. Or vinyl cement.
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