Powder coat would probably work but may be overkill for the problem. Eons ago when Chrysler was building big performance V-8s , they made some magnesium parts. And if you ever thought cast aluminum was porous.... Anyway, Chrysler Engineering began going down the street to a Rexall Drugstore and buying pint bottles of egg preservative solution, also known as sodium silicate or 'water-glass'. The porous castings were taped up, filled with the solution, drained and air-dried. That's all it took to seal them up for pro-racing, and the excess solution was reuseable. About $3 a pint.