My favorite poultry waterer is a two-gallon vacuum model made by Miller Manufacturing. It can't be used with apple cider vinegar (two tablespoons per gallon is often used for chicken health). I have plastic waterers for that.
Unfortunately, my waterer soon began to rust. This is how it looked after about nine months of more or less daily use.
Online, I found a few possible explanations:
1. Faulty manufacture
2. Hard or acidic water
3. Adding vinegar to the water
4. Cheap construction, aka, they don't make them like they used to
I could rule out 3, because I had avoided that. I had well test data for 2, and it was a possibility. My old well water had a pH of 6.2, and the new well's water is still acidic, although less so at 6.6. Both wells produce what the reports called 'slightly hard water.' Miller Manufacturing very graciously replaced my waterer but suggested that my water was probably to blame.
On backyardchickens.com, some people indicated that they had older galvanized waterers that had no rust after extensive use, but that new ones had rusted rapidly. They believed in their own cases that 4 was the culprit for their new ones that had corroded.
I bought an old galvanized waterer off of eBay of uncertain vintage and found it soon showed signs of rusting, too.
So I decided to adapt an idea I saw on backyardchickens.com by user dcwatson84--I got some pond liner epoxy and rust treatment. It's pond safe, so shouldn't pose any toxicity problems.
First, the rust needed to be removed. I used Evapo-Rust, which worked well, although it was very slow. Because I didn't want to buy a sufficient quantity to immerse the waterer, I soaked paper towels and placed them over rusty spots per the directions. It took up to 24 hours for the rust to (mostly) disappear, but most areas had to be saturated a couple of times because some spots in treated areas didn't have good contact with the towels the first time around. It took a week of daily (or semi-daily) towel soakings and changes.
Then when I got the epoxy, I was surprised to see that it called for using an 'etching primer' and pre-sanding with 60 grit. I had some 150 grit and wasn't coating a comparatively rough-surfaced pond, so I decided to give the 150 a try, sans primer (I had looked for some immersible paint or primer and only found Rust-Oleum 8400 [>$100 per gallon] as a possibility). Because of the former rust, I reasoned that it was already somewhat roughened, anyway.
Unlike the original poster, I did not coat the whole thing. The epoxy is expensive, and if limited to water-contact areas, one 1.5-quart ($50! 😢) set should be enough to do several waterers. Plus, people have in the past suggested that I am exceptionally cheap-another said something about being so tight that twanging in the wind was a possibility. So mostly just the interior was coated, though I also did the outside pan--even though it did not have any rust (yet).
I also did not remove the rubber stopper but tried to paint around it. Some epoxy ran down and stuck to it, anyway, so I carefully cut around it with a utility knife after the epoxy cured. The result functions about like it did pre-epoxy.
Although it looks wet, it is dry in the picture above. It dries glossy. You can see where it's not very thick in some areas; it remains to be seen if rust will occur there. This does not seem to have hurt and may help.
I have three more metal waterers--two that are new (the replacement 2-gallon one) or relatively new (a 3-gallon waterer that has seen only limited use and so far has no rust). For the new ones, I plan to prophylactically sand and epoxy those, as well. The eBay one that is now rusting will need to be pre-treated to eliminate its rust.
If I get three done with the $50 worth of epoxy, that's an extra $17 (roughly) in each waterer, plus a significant time cost (or $12.50 with four waterers done; I doubt it could do many or any more than that). That may or may not be worth it going forward, even before questions of durability of the epoxy coating are answered.
But there is one bright side: the chickens are showing no ill signs after drinking out of it for a while. I assumed it'd be safe, but one never knows for sure in advance...