You know...that almost looks like a fuel tank for a boat. having had boats, and working around boats all my life, yeah....it definitely looks like a fuel tank in which case it is most likely stainless steel which would explain the magnet not sticking. I am not 100% sure, but my best guess would be a stainless fuel tank that some guy painted not knowing the properties of stainless steel ( boat owners.75% clueless...lol). Let us know please.