I seem to have a badly corroded seal flange when removing my exsiting toilet to have it replaced. The screws are very rustly. Anyone have any infomation on what is typically underneath? As is are the screws just into the fiberglass or into the flange on the black water tank? I could probably section the flange seal off but I don't want to remove the heads of the screws if they are screwed into the flange. Dose anyone have any suggestions on how I should appraoch this?