Dont bother, its not worth the rish on a valuable tank. Get a proper tank pannel beater will get it out via the filler hole or make a small hole or window in base of the tank and get it out that way. The good ones will be able to repair it so you wont even know/see it has been opened at the bottom. Other option is if its a shallow dent and a few ml's reduction in capacity is not an issue for you then there are modern hi-tech body fillers available these days that will fill the dent fine and the so called 'bog' is very good and wont fall out like perhaps it did in the old days or its more due to the fact that the person probably didnt know how to apply it properly.