I've been using Evapo-Rust for about a year to clean up rusty tool parts. I've had a gallon that I would re-use and it's finally died on me - what happens is that it gets to the point where it's not getting into the pits (if the metal is pitted) and after wiping the part flash rusts pretty quickly. Looking back over the documentation it states that a gallon will treat 300 pounds of metal. I can attest that I've put way over that amount in the gallon I've been using (I filtered the liquid when there was too much sediment). When it starts to fail it creates a yellowish/greenish slimy sludge that sits on top of the part in the bath - this stuff wipes off and I'm assuming that it's a mildly sulfurous compound (no eggy smell though).
Some tips:
1. Make sure you have enough to submerge the part completely or you get a watermark from the dip - this is almost impossible to remove.
2. When not using, reseal in the container as I believer there is some dilution with exposure to air.
3. My process is to dip, usually overnight as most of the parts are heavily rusted, remove next day and wipe down - if there's a lot of rust I'll hold it next over a garbage can and scrape a bit, which removes the heavy stuff - use a wire brush, sandpaper or razor blade. If it's still rusted put it back in the wet. When you're satisfied that the rust is off, re-dip and let air dry - it keeps it from flash rusting (says 2 weeks but it actually lasts longer).
4. For irregular shaped objects I place 4 mil plastic in a plastic over-sized tub, then shape the plastic to conform closely to the part - put a couple of blocks of wood underneath so the plastic isn't sticking to the part or you'll end up with water marks areas not derusted.
5. Best price I've found is at Harbor Freight - they have gallons for $20 and you can use a coupon (I get 20% off coupons in the mail or via email).
6. More info here: http://www.evaporust.com/
-- John
