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I would use a grinder also but instead of a grinding disc/wheel I would use a flap disc. You will have to take your time so you don't tear up the disc but a flap disc leaves a much smoother finish.
Honestly I am not sure what all grits are offered in a flap disc but I would try to get something in the 125 grit range. I buy mine at the local welding supply store, they are a little more expensive but are a lot better quality than those purchased at a local "discount" store.