Are most gas tanks made from mild steel? Is there any reason why they're not made of any other material that is lighter and doesn't rust? It just seems to me that tank "technology" is a little bit behind since they're heavy and prone to rusting. I was thinking... "Why not make gas tanks out of plastic?" but then I realized that perhaps it's a safety issue... Plastic probably cracks easier and you really don't want your fuel spilling all over the place in the unfortunate event of an accident. But still... Why not aluminum? Or even stainless steel?
And on a related note... If your tank is completely rusted I think you can do something called a "hot tank". Does anybody have any more infortmation on this? Where it can be done and for how much?