Yikes! Please don't use Rustoleum in chicken waterers and birdbaths! Aside from the fact that Rustoleum and other anti-rust paints are not intended for use in food/water containers, and may leach toxic chemicals, these paints contain high levels of zinc, which can cause zinc poisoning in birds. Here is a link on heavy metal poisoning in birds , which cites two instances of poisoning in macaws/parrots after being exposed to anti-rust paints.
The same is true of deteriorating galvanized coatings on metal containers -- they can leach zinc and other heavy metals into the water, particularly if exposed to acids and strong chemicals, such as vinegar and bleach. When rust forms in a galvanized waterer, this indicates that the galvanized coating is unstable and is wearing off, and it may be exposing your birds to chronic levels of zinc that may not be healthy, so it would be best to replace that waterer. As a rule, it's best to not used caustic or acidic chemicals in either plastic or metal containers, as these will cause the containers to leach estrogenic-active chemicals and/or heavy metals, including zinc, lead, chromium and cadmium, all of which accumulate in the body tissues and can be passed into the eggs in trace amounts. For serving vinegar supplements, the ideal is to offer this separately, in a ceramic container with a lead-free glaze.
On a related note, old timers used to add rusty old horseshoes to their hen's drinking water. From the February 1908 Farm Journal: "If you have a scrap of rusty iron, put it into the drinking vessel. There is no better tonic than iron water."
This advice was echoed in the Hagerstown Town & Country Almanack of 1910: "Rusty iron placed in the chickens drinking water vessel acts as a valuable tonic."
Whether or not adding a rusty old horseshoe to a chickens' drinking water is good advice, we can at least rest easy knowing that small amounts of rust in a chicken's water are safe.
Edited by Katzooks - 6/9/11 at 6:29am