Removing rusted screw and wingnuts from toilet tank
When I had the same problem, I took a 4 1/2 inch angle grinder to the bolt heads. Just be careful and try not to touch the grinder to the porcelain. Also dont grind so long that you heat up the bolts to much and be careful not to melt any plastic bits in the toilet with the grinder spray. When you are done grinding, make sure you clean out all the slag and dirt that you generate. If you only grind inside the tank, you dont have to worry if you slip a little, any marks you make will be hidden when you close the tank lid!