Field Application of Zinc Phosphate Conversion Coating

A. It is not practical to do it in place. A valid substitute would be to clean the valve,blasting preferred, and spray it with a can of "Cold Galvanizing Compound / Zinc Rich Paint". It is a high zinc content spray paint that looks like aluminum paint when applied. The zinc is sacrificial. The stuff bonds well to a clean part. Put one or two coats of a good paint on top. Since you are a waterworks, I would use an enamel.

A. Try any proprietary rust converter [on eBay or Amazon] (do it yourself solutions can be used too). As rust remover you can use 5% ammonium citrate solution gelled with some wallpaper glue(cellulose based). After treatment surface must be rinsed well and then painted or lacquered.
Good luck and hope it helps!

A. Although I agree with James in that it would be easier to paint it, there is a brush plating technique designed to zinc plate on site. Obviously it has pros and cons, do's and don'ts. If the recommended painting proved insufficient you may want to investigate this technique, also referred to as selective plating.

A. While brush zinc plating should be superior to cold galvanizing, the original question was "phosphate". I feel that cold galvanizing is better than plain phosphate. Cost of equipment and training to do the job would be high. You probably can replace the valve for less. I am moderately sure that there is a brush plating contractor within 100 miles, but it still is not going to be cheap. So, I think that the economical way to go is the cold galvanize, even long term.

A. The trick to zinc phosphating something in the field is to coat it with the solution and then cover it with a damp cloth to keep it moist. If it is done properly you will get a nice gray matte finish.