Seal all the holes except the filler. Toss in a hand full of square nuts and a few bolts in there. Make sure to count them going in. Rattle it around a lot. Pour out the same amount of hardware. Rinse with water. Get some Red Kote. I believe Auto Zone carries it. Both in quarts and gallons. By the gallon is cheaper. Follow the directions on that Red Kote. A gallon will do more than a twenty gallon tank and runs about fifty bucks +/-. After I rinse mine with water I usually finish clean it with MEK just before the Red Kote, making sure the MEK is dried out first. I did a neighbor's tank about six years ago. We had to screen patch some holes with JB Weld. Painted the outside when done. Looks good. It was almost a basket case. The sediment bowl filled up in less than five minutes run time with tank rust. Now it's crystal clear....