My truck arrived with a tank half full of very rusty water. I drained and flushed it. With nothing to lose, I put a shovel full of sand and gravel and some more water and shook it hard and rinsed more water through until it ran clean. This took some time and strength. It came up clean enough to use, but ongoing rust would be a problem.
Its been sitting dry for a while , while I restore the truck. I expect to use one of the available tank-liner type coatings. I have used them before in motorcycles with good success.
