Muriatic acid in a 50 percent solution of water. Makes no sense to grind away good metal, does it? Acid will convert all rust and etch. Clean with soap and water and immediately coat with zinc chromate primer after rinse has dried.

If you have doubts..try 50/50 in a coffee cup and immerse the most rusted bolt or small metal piece you have..wait 2-3 minutes..then remove. The good metal is left intact.