Hey Scott,
I tried EvapoRust in my 750F tank without much luck. I followed the directions for the tank being off; that meaning one gallon of product in the tank with rotations every couple hours. I left it in for one day on each side (bottom, right, left, top, front and back) and was not impressed with the results. I still had rust and heavy pitting that wasn't dissolved.
Some will say that prep is everything and I agree. I cleared varnish using Dawn soap and very hot water in combination with a chain to break up deposits. After soap and rinse I saved with acetone and dried for a few hours. Then the gallon went in...
I feel as though a full tank of Evaporust would have done the trick but not at $25 a gaĺlon. Maybe some guys dillute the solution but it wasn't something I did. Vinegar up to the brim, a baking soda bath after, and a bit of prep and etch to catch any and all residual rust. Worked like a charm but I found pinholes and now need to seal.