The coating on this popup toggle bathtub drain is damaged, exposing the plastic inside and I guess there's some oxidized metal there. It works fine though, so could I clean it up and re-coat it somehow without too much work? Or is it just simpler and/or more cost-effective to get a brand new replacement?
The answer depends on how much work you want to do. In reality it probably isn't worth the time to try to recoat it. however if you wish to do that there are epoxy paints that may be suitable. Time is needed to clean and sand the oxidation off and prep for the paint.
I would unscrew the pop up. Buy a new drain. clean the base left in the tub and screw in the new pop up.
There, an almost like new drain.
You could just sand the place where the chrome plating is missing and repaint. You could just dab some paint on without sanding. Plenty of people would just tolerate it as it is.
As another answer states you might be able to unscrew the pop-up by hand to take to a plumbing supply and get another that would work. The tub would be usable as a shower without a pop-up in place, but it opens the drain so that something could fall in and be lost or block the drain.
The entire drain in the wet area could be replaced, but this is not easy unless you are experienced. There are videos showing how this is done.
It can be done entirely from the user (wet) side of the tub without access to the underside of the tub. The parts of the drain below are kept. You have to unscrew the old drain to see what is the thread pitch because there are two different thread pitches used (coarse and fine). The new drain must have the same diameter and thread pitch as the drain under the tub because that is going to stay. You can determine the diameter of the drain with the drain in place, but not the thread pitch. Experts here may know that one or the other of the thread pitches is more commonly used.
It appears that the tub is rusting around the drain hole. When you have the drain out you should carefully sand or grind away the rust down to bare metal and use the proper paint before you install a new drain. There may also be rust on the underside of the hole and you will have to inspect that with a dental mirror to see how much rust is there.
