Well we know the "right" answer is to replace the tub, but we all have to live with some other reality some time, so here goes:
1. There is a rubber gasket on the underside of the tub between the drainshoe and the tub bottom. That has to be replaced anyway, so reach in and pull it out. Use some sandpaper to gently clean the underside ot the tub around the hole.
2. Clean up around the rusted hole with some fine sandpaper. Get some good epoxy putty such as Rectorseal 400. Put some masking tape temporarily on the underside. Fill in the hole, using a plastic scraper to make the patch more or less flush with the tub surface.
3. When the epoxy sets, install the new rubber gasket on the underside. I would coat this on both sides with some silicone caulk; use clear 100% silicone caulk on the underside of the shoe as you screw that back in. One the shoe is tightened, if any part of the patched area is visible, I would ensure that some silicone covers it completely. Let all this set up.
4. If this was a big leak and you have opened the ceiling underneath to repair drywall, I would add this step, which is a top secret plumbing trick not to ever be used unless authorized by deep throat: I would take a brush and completely encase the underside tub drain shoe area with Henry's 208 roof patch (tar). You must understand that this is a permanent and non-reversible step. Your next move if there is ever any other problem is to remove the tub ( or add more Henry's). Most folks here will probably criticize this step. I have used it many times in just this circumstance where it is not possible to do the "proper" repair for some reason ( usually financial) and you just want to make sure this does not leak for a long time. It has never failed me. It doesn't look pretty,but sometimes..............
