I am looking for a corrosion protection "coating" that I can apply in my shop to protect magnesium aircraft castings from corrosion by water at about 100 F.
The proper procedure includes cleaning, pre-treatment, and coating. I doubt that you can do all of these things in your shop. Be that as it may, a typical process would consist of alkaline cleaning followed by acid cleaning (nitric-sulfuric acid or chromic acid). There are numerous pre-treatments that exist including chrome pickle, sealed chrome pickle, fluoride/chromate, anodizing, etc. Coating would then typically be an epoxy primer followed by a polyurethane topcoat. An excellent paper describing the development of a highly-corrosion resistant coating system for a magnesium helicopter transmission (anodizing + epoxy sealer + epoxy + polyurethane) is available on the Tagnite website (anodizing vendor). Use the following links for more information, or obtain a copy of ASTM D 1732 Standard Practices for Preparation of Magnesium Alloy Surface for Painting or MIL-M-3171C.
