I'm preparing to home manganese phosphate my new Yugo barrel for my kit, and mostly have used the 2 following links for my info (two threads from same site):
Home brew parkerizing
Manganese phosphate finishing
In the links you will see a picture of Jasco etching phosphoric acid someone has used.
After some hesitation (because it looked soapy in the bottle) I picked up this at my local Home Depot:
It's less than $15.
This was just a 1st experiment, so I did very rough approximations but put a small amount (less than 1/2 oz) of the above etching liquid in about 1/2 quart of tap water, added a dime-size piece of steel wool, about 2 teaspoons of manganese dioxide powder (pottery supply shop) in a stainless pot, and heated to 175-185 degrees.
I did varying levels of metal prep on stuff I had lying around, and put each into the pot about 20 minutes each, or until they stopped fizzing. The liquid first fizzes the steel wool until it reaches some saturation, and some recommend you season it further with small metal pieces before you put your best metal in. That seems right because although the first things I put in were the best-prepared with degreasing and bead blasting, it was the later peices that were progressively darker.
Below on the left are three nails. The first (left) one is how they started. The second was bead blasted prior to parkerization. The third I only sprayed with carb cleaner and wiped with paper towel prior to manganese parking. The middle nail is darker here because the zinc or rust on the right-hand one converted to blue-green powder, almost the green cosmoline finish some people strive for.
The pry bar on the left and the needle-nose pliers were both bead-blasted, and they were the first peices in. In spite of their better prep, they came out lighter than the wirecutters, which I very briefly blasted. They were the last in and came out much darker. All tools were rusted bright steel prior to this. Sfter a hot water rinse, I wiped on a light oil and then wiped off the excess.
This was a sloppy first go just to see if the $12 etching liquid was capable of this, and I'm fairly impressed with the result. It probably took me 90 minutes total and cost $15. A bit cheaper than the $250 kit I was considering, and that bottle of liquid is enough for 128 gallons of manganese phosphating liquid!