I have used 3 different Brownells parkerizing products with generally good success so far. All were a liquid you diluted in distilled water.
Many years agoI used "Amerlene Grey" to park in a friends gunshop, he used that exclusively for a while. It worked well, but we usually batched several guns together to amortize the cost over several rifles. IIRC, it was about $15 to mix up a batch and it was not supposed to be reused once it cooled. Never tried to reuse it so no clue if that would work or not. We did use the pre-dip blackener, IIRC it was just some acid you mixed with water in a plastic garbage can to dip it in. Stuff did turn out black with it. The normal finish was a light to dark grey. I recently saw a pistol I parked about 15 years ago, the grey finish was somewhat worn but it had held up well and was beginning to exhibit the desireable green tint found on vintage weapons. The owner had done nothing but oil it occasionally.
As his business in refinishing tapered off due to poor health, my friend tried the Brownell's branded Manganese solution as it could be reused multiple times. He often did single guns with this solution and I helped him out on occasion. The finish was usually a very dark charcoal grey to black. We never used the predip black on these as the color was black enough without it.
Recently I bought a bottle of Brownell's branded zinc phosphate parkerizing solution. A gallon was about $30 and makes many gallons of solution. It costs about $5 to make up enough to fill my friend's tank and it can be reused many times. Nice even grey color about like the Amerlene provided, but the shade varies somewhat depending on temp and how many times it has been used before. Still experimenting, but I like it so far.
My friend is long out of the refinishing biz but he still does have most of his equipment. He uses a TIP blasting cabinet system that works very well. IIRC, they run about $900 but are a professional setup. A smaller cabinet will work fine as long as you can fit your work inside properly. As TK said, the compressor is critical. Most blasters will suck up at least 8-10 CFM. The compressor I am using now is only rated at 6 so I have to stop often and let it build back up. An annoyance for a home user, unacceptable for a pro. Wish I could afford that I-R 13CFM jobbie down at the TSC store.