How big is the dent, and how deep is it ?
If the paint is cactus, some dents can be shrunk out, using the CAREFUL application of heat, and either a damp rag or some dry ice, depending on the severity of the dent.
NOTE that heat and petrol tanks can be a dangerous combination.
I've also soldered some (many) tags onto the area of a larger dent, and used a slide hammer to gently pull everything back into shape - learn't that one at tech classes. After, you unsolder the tags, and can file finish the surface, the solder should come off completely. For not too deep a dent, this can work well.
A really large dent can be hammered out by cutting a hole in the base of the tank, as mentioned above.
For really deep or bad dents, this is about the only way.
After, you oxy the piece removed back into the base.
Only attempt this if your oxy skills are good, and all the petrol is GONE.
I prefer to give this to guys that do it regularly, it needs to be leakproof.
And it can take quite some skill to get the required shape back ...
