Thanks for the help, folks. While it sounds like the rust may be more or less safe for them to ingest, it's just spreading so quickly now that I want to stop it from progressing. I'd like to keep using the waterer for a long time.
Since I already have some Rustoleum kicking around from various...
I've been using a 3-gallon galvanized waterer for 2 years, and very recently, it developed some nasty rust spots on the floor of the inside, as well as around the edges of both the base and the cap thingie. If I don't scrub it out and refill it at least once a day, the water they're drinking is...