Evapo Rust is a really great product and works very well however it is pretty expensive. So dont go doing what i have done.
In the FAQ's on the Canadian Manufacturers site it has this note:
"EVAPO-RUST™ has an indefinate shelf life*
then the * down in the fine print says
*Shelf life is for un-opened product. Once opened, The life of the bath will vary due to biodegradability (bacteria introduced into the solution). After using Evapo-Rust, it is better to cover the solution, but still allow air to the bath. Pouring the solution back into a sealed container will allow the anaroebic bacteria to begin the degredation process.
Used, open baths have continued to operate for over a 12 month period.
At $35 per gallon up here (what a price for water with some chemical mix) this is an expensive lesson.
To me this is a huge downside, reason being its fine if you are doing old iron restoring etc continuously then i would keep a bath. But if you are doing the odd tool restoration that could be say 6 -12 months apart then who would want a bath of water based chemical in their shop for the evapouration of the water to go about creating more rust.
So either way seems you take your chances.
The good side of this stuff is the way it works and the protective coating it leaves when you remove a part and wipe it down. You dont get any of that instant rust effect.
So my new lesson is buy as little as possible, meaning get your container very appropriately sized to suit the part, then dispose of when done or store back in the container and hope for the best if its done when you try it again, well you were in for new stuff anyhow.