I have dealt with dozens of rusty tanks and you should not over think this. I would only use the Kreem if a replacement tank was
unavailable, very expensive or if the original tank was necessary to maintain some restoration type project.
First remove the tank and rinse out as much loose rust as possible. Remove the valves and screens if any and plug the holes.
Now pour in a couple cups of muriatic acid. It may fume, use protective gear.
If it doesn't start reacting, slowly heat the tank. Set it in strong sun on a black surface or play a torch on the bottom.
It will certainly fume at this point, don't breathe it. Agitate the tank gently and let it work for about 15 minutes.
Rinse the tank several times with water then once with acetone or rubbing alcohol to remove all the water.
Look inside and it will be shiny clean and have the same surface it left the factory with, bare steel. Bolt it on and start dredging.
Kreem type products are fine and do a good job but for a dredge motor they are complete overkill, time consuming and expensive.
It you didn't live on the opposite side of the planet I would give you one of the tanks I have sitting around here
