didn't know you got it second hand... hopefully that didn't have something to do with it.
I've been more than happy with my IMS, and it's been bashed many times, as I'm not the greatest off road rider. it even lasted 1000miles with my radiator guards rubbing the tank before I trimmed the guards down. very little mark made.... My model is quite thick, though in the location of your crack, I'm not sure of the thickness...
I'd be willing to bet you got a bad tank... I'd give IMS the chance to replace it.... I doubt the paint will cause an issue, as most advertisements I saw stated "paintable" so I don't think they could cry foul...
but hell yes... good thing that shit happened now!