I'm having problems sealing a garbage disopal sink flange in my kitchen sink. The sink is a coated iron or steel sink. The hole in the sink is downward curving, not flat across like a typical stainless steel sink. As such, the lip of the flange does not sit flush on the edge of the sink hole. Further, the sink hole is a bit larger than the flange diameter, which allows the plumbers putty to escape under the sink as the flange mounting screws are tightended. It seems like the gap between sink and flange is too much for putty. Everyone recommends plumbers putty for sealing between the flange and sink but in this case water inevitably gets down into the crack between the flange lip and the sink and slowly makes it's way through the putty. Any suggestions on how to solve this problem? Is there a special rubber gasket for use between the flange and the thick, downward curving sink hole of a coated iron or steel sink? Is there a special shaped flange for the downward curving sink hole of a coated iron or steel sink? All flanges I have seen appear to be the same. Thanks.