If you go the Nitric Acid route, you will want something between 5 and 10% HNO3, but put the nails in the water, THEN add the acid.
At 10%, you will want 9 oz, of water, ONE ounce of the acid.
Mixing acids into liquid, or vice-versa, it will cause an exothermic acid - lots of heat generated. So always remember Triple A - Always Add Acid. Or base, also. Like Sodium Hydroxide, same effect.
Sometimes the Nitric Acid will cause a lot of heavy orange fumes, so do your mixing outside, and wear rubber gloves and some sort of eye protection, even sunglasses will help if you haven't safety glasses.