The 2 things im guessing when you say the geometry is off is either A) the cutting edge isnt square to the body of the chisel or B) the bevel on the chisel is wonky in some way. 2 different problems ive hit with 2 different solutions that work for me.
A) Take a fine point sharpie and a square, draw a line across and grind to the line. This usually moves the cutting edge back too far for a normal sharpening to take care of, well, sharpening, leading directly to...
B) Regrind the bevel. Despite my usual hatred for hollow grinding, i think a bench grinder is the best tool for the job here. You could probably use a low grit stone, and if you do, once youre done get back to us on how well it works. Admittedly, youll probably have to tell our children, as well all be dead, but still. Anyway, whatever method you choose, find a way to set the proper angle for the chisel and keep grinding until the proper angle youre grinding shows its way across the entire bevel. Its pretty hard to describe what im trying to say with that, the best thing i can say is youtube is youre best friend. Once youve got the bevel re-ground, sharpen as normal.
All said and done, ive never seen anything that permantly destroyed a carbon steel blade, provided it isnt cracked in half and lit on fire. Ive heard good things about sheffield, so i have no doubt that, given the proper care, youll be able to have the edge cleaned up and cutting in either no time flat or a life time