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?