I used this chemical...
Receipt from Paul Johnston (PJ's Gun and Metal Care Products)
You use 1oz of Parkerizing chemical for each quart of distilled water, I used 1 1/2 quarts for the pistol. This is the "lighter" zinc phosphate, which give the light-med gray color. If you use the manganese phosphate, you'll get a darker-black color. The chemical is very easy to use, hardly any fumes, no damage to anything, easy to clean up and relatively effortless. Cleaning, degreasing the gun is most important. I believe the 70-80 grit aluminum oxide media the sandblaster used was the right choice for surface prep.