I do not like to introduce any water to the zinc, so I wash the castings with naphtha. Then the casting is wiped and air blown dry. Then I spray the castings with Triflo, which is a light oil with Teflon powder in it. I scrub the oily castings with a bristle brush. The fine detail is cleaned with small head cotton swabs on wooden sticks. These are available from McMaster-Carr. Then the casting is blown dry and dried with paper towels. The areas that are heavily corroded with either rust or zinc oxide will be lightly wire wheeled with a Dremel stainless brush first. This method almost always removes the white corrosion and restores the black finish. I have been using this method for many years and have never needed to repeat it.