I recently found an incomplete set of dull, rusty spade bits that belonged to my father. They are likely 40-45 years old. Are they worth trying to clean up and sharpen or should I just throw them out? I don't have a grinder so I imagine it would cost more to have them sharpened than to replace them.
I have had mine for quite some time and have sharpened them many times with a hand file.
Just be careful how much you remove on each side and keep the angles consistent for proper cutting. The thin metal of the bits would heat up too much under machine grinding or the angles would not be easy to reproduce, IMO.