I tried some of the putty stuff on my MGB which had a crack in the tank at the edge where the sides joined up. Didn't do crap. The stuff would not stick to it because it was wet with fuel.
Tried the same stuff to try to fix a water leak on a hose fixture, and the water pressure sent that stuff flying.
Had the gas tank in the MG replaced as welding was not safe to do unless I had him drain it, dry it, weld it, etc etc. Was easier just to replace the tank and I had more peace of mind.
I have seen many stereo shops drill holes in people's gas tanks when mounting amplifiers etc, without realizing that they did that.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.