My 54 6400 has a fuel tank that is welded to the out side of the frame (it is actually a fuel oil tank). It has been sitting for years with nasty fuel in it and about 1/8" or more of gunk in the bottom. There is a 5/8" drain plug in the bottom so I can drain the nasty fuel but that still leaves the gunk. Anyone have an idea on how to remove the gunk while leaving the tank in place?