Done ospho and POR15 ... dont like both entirely but for different reasons.
Here is some examples of my handiwork.

http://picasaweb.google.com/srinath.the.man/SmithvilleFlatsTank#

OK I am yet to take pics of it with ospho in it.

But ... ospho will leave a lot of ashy powder in it and scaled up phosphate everywhere. Chain may not get it off.
POR 15 is so particular about the surface being totally dry and rust free.
The answer ... OK depends on the tank ... I prefer POR15 after muriatic acid on a tank I will have to powdercoat. I prefer Muriatic acid and Kreem if its been powdercoated already.
I will do ospho only if the tank has no corners or crannies, not a savage tank ... virago 1100 tank yea. ospho and POR15.
Cool.
Srinath.

You can take the chemical approach and use aircraft stripper, or some of the other heavy duty commercial paint strippers. After several hours it should come off with scrapers and scotchbrite.
Be cautious of the container you use with chemicals. Lots of these strippers will also eat thru plastics.  A large metal container is best. Also, do it in a good well ventilated spot away from heat sources.

You should be able to find a local shop that will sandblast one for you for $20. By the time you're done buying chemicals, scotchbrite, sander attachments, etc you could spend more than that.

Paladin, check yer PMs Man..