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Regarding rust removal, first remove any dirt, dust , or oil. Washing in dish soap works fine for this. Then I simply use a plastic bin and some strong white vinegar. Evap-o-rust works great too. Toss the parts in and let it set overnight. That’s typically long enough unless it’s really rusted. After that, use a wire wheel to clean away any remaining rust. Rinse off with water, but then immediately dry it and coat it with something like T-9 Boeshield or a rust protectant to prevent flash rusting.
For flattening, I apply adhesive sandpaper (100 grit) to my table saw table. Make sure the blade is removed. You only want to sand the sole. Reduce the grit until as smooth as you want. Flatness is more important than smoothness.