The problem is its producing a ton of rush. Any ideas on how to treat the rust or stop it, or am I doomed to have them rust out? I went with metal as it absorbs the heat from the sun much better than plastic.
I am not aware of any product to stop rust forming. I could be wrong.
I think you should expect these barrels to be good for a few years.
Long enough to see if they do the job for you.
At that time I would replace them with the plastic barrels. I would think a black plastic barrel would do almost as well as the metal ones.
I run a micro hydro. The first 5-8 years I used several metal barrels as my water dump. About 2-3 years before they rotted thru.
In the 20 years since then I have bought one plastic barrel and it is still as good as the day I bought it...