I coat and polish my housing with silicone grease to prevent water spots and damage.
Careful drying of the glass surfaces, viewfinder and housing immediately after each dive is most important to prevent water from drying and forming deposits or white spots.
Before a dive trip, I lightly coat the housing with inexpensive silicone lubricant, the kind sold for swimming pools and mechanical use, and polish it with a soft cloth. The silicone protects the finish, fills in microscopic pores and gaps, and gives the housing a nice glow.
After diving I dunk the housing in cheap vinegar. I buy a gallon (4 L) of inexpensive cooking vinegar, and after diving, sink the housing with viewfinder and port in 50/50 vinegar/water for a day or so. This dilute vinegar can be kept and reused.
Once dry, I again lightly coat the housing with the inexpensive silicone lubricant and rub it with a soft cloth. My Nauticam and Subal housings look like new, even after several years of use.