We have a cast iron tub that had a little rust around the draining hole. I tried to fix it by using wire-brush/sandpaper and sealed it with porcelain touch-up glaze. But after 4 month the thin rusty line showed up again:-(
Any suggestions how to fix the problem.
The best way to tackle this is to remove the drain and then do your "brush, sand, and touch-up". If you don't remove the drain there is still rust under it that will migrate back into view. Apply a number of thin coats of glaze before reinstalling the drain.