Remove drain and thouroughly clean it. Check to see how bad rust is. Clean off rust and loose enamel. Apply rust converter to rusty metal (phosphoric acid) and let it do its job. Wipe than rinse off when done and dry completely. Rusted area shound now be black/gray. Apply an epoxy resin based enamel and let it cure. Apply silicone around the drain pipe and reinstall drain. This should be a long term fix. 😁❤