When deciding on the best procedure to use it is worthwhile to
determine what has caused what you trying to remove. Rust
and corrosion is a result of moisture so water base solvents
usually produce the best result. Goop from dried up oil and gas
responds best to solvent base cleaners. If your dealing with both
removing the goop first and then going after the rust and
corrosion seems to make the most sense but a water base cleaner
that dissolve grease can be effective with time. If the tank is so
rusted or corroded you end up with holes in it you should search
out another. Even if you patch the holes in some way the tank is
weak because metal is missing throughout the tank and it can
puncture easily.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.