18 ga cold roll steel
start at the beginning of my build it only been about about a month so it not long. The tank bottom in the build will not be used.
start by cutting a rough shape, heat the metal to take out the temper, to about blue, let it cool as slowly as passable
I use a wood box full of sand us a heavy object with no sharp edges and beat the tar out it, beat it full of knots.
for less than $200 you can get a harbor freight planishing hammer and stand, it works very well and if you are not afraid to us some bondo to smooth it out it will get it good enough.
as far as the edges, the method I us hanging body panels is the same way I mated the tank panels. Take the two panels you want to joint, over lap them slightly tack them together then us a die grinder with a thin slicer blade and cut through the edges of both pieces and you have a perfectly mated edge.
mark