Tust around tub flange/drain

I have a standard tub but there is a growing rust spot in the tub right by the chrome drain flange (not on the flange itself). I want to take out the flange, sand down the rust, seal with enamel epoxy, and then re-install the flange. I have no access to under the tub. Can this be done? I've installed drains in sinks before but that was always from underneath. Can I remove the drain flange without access to under the tub? How? Can I re-install without creating a leak - will plumber's putty seal it? Will sanding down and coating the rust work to stop (or at least slow down) the growing rust in the tub?

Sounds like a steel tub close to the end of it's life...
That would have Do Not Disturb written all over it unless you are ready to remodel now.

tub

If it is a failing cast iron tub, (and it would be rare if it were), and the drain plug is not frozen in place because of a poor installation, you may be able to do it. But if it is a steel tub, when you remove the plug you may find the entire area underneath it has rusted away and there is no way to repair it.