I used it on my TBS tank - it had a fair bit of internal surface rust. Temperature is key - the higher the temp, the faster it works. I ended up by putting it in a closed bathroom with a space heater and what had been going slowly at 65 - 70 finished very quickly at 85 - 95.
I used 3 gallons in the tank and rotated it every 3 - 4 hours, to get everywhere. Total time was 48 - 60 hours but would have been much faster if I had learned just how temperature dependent the reaction is. I did try to start with 1 gallon but went to 3 to get the job done faster with better area overlap given that you can't really see well into the tank.
My rust wasn't deep but was fairly extensive. Once done I rinsed with 90% rubbing alcohol, dried as best as possible overnight in the closed room with space heater running then replaced and filled with gas.
I had a hard time sealing the petcock hole and filler hole so the outside tank and paint were bathed in the evaporust and absolutely nothing bad happened to the paint, it is paint safe. Works as advertised and now have about 2.5 gallons back in the original containers for the next time it's needed.
I didn't coat the inside of the tank once done and haven't had any rust return. I do try and keep the tank full; even the rust on the filler neck hasn't returned. I did my tank almost 18 months ago.