Cleaning out old fuel barrel

In shipyard repair we used to clean fuel tanks with Tide detergent and scrub brooms. In your case I assume you have no access hatch or inspection plate to get into the tank easier so what I would do is remove all vents to the tank and add a large box of detergent, and fill the tank in small increments and use a pressure washer wand or even a small diameter pipe or tubing from an air compressor to spray air through the solution against the bottom and sides of the tank. As each level is cleaned and the sludge is loosened up you can add more water to go to the next level--maybe in 6" increments or so. Most of the sluge will be on the bottom of the tank but if you had some algae in the fuel, there could be a bunch of trash hanging down from the top of the tank where the condesation would have been. You can drain out the tank as needed and refill with more detergent and water. Naturally you'll want to follow all local state & fed regs for disposal of the wash water. When finished cleaning, finish up with a good fresh water rinse with the drain open. When you get a clear stream, you'll know that the tank is clean. Won't hurt to bang on the tank with a 2 or 3 pound maul and a 4"x4" to loosen any stubborn grime. When I recently cleaned out a diesel tank (right after the hurricane) I set up the tank opening in front of our diesel generator exhaust for about an hour or so and that completely dried the tank's interior. If you can't do that, then let the tank stay open a few days and it will dry. Finally, you might want to spray some clean diesel on all the surfaces before you fill the tank and watch the drain contents--should be nice and clean.

It would be easy to just refill the tank, but one thing I've learned about filling with water--you are performing a good hydro (pressure) test on the tank boundaries and its better to find a leak with water than with a fresh load of diesel after the truck leaves.