(got this off falfiles.com i think)
ZINC PHOSPHATE PARKERIZING
Phosphoric acid can be bought as a primer for bare metal. It is sold under the names Ospho, Metal Prep, Ph-ospho-ric, and Jasco Prep and Prime, and is a greenish liquid. Take 4 pennies, dated 1990 or later and drill small holes in them. Heat up 8 oz. of the phosphoric acid in a microwaveable beaker or cup. Drop the zinc disks (pennies) into the hot phosphoric acid. The pennies will start fizzing and leave them in for about 5 minutes. Remove the pennies from the acid. SLOWLY transfer the liquid acid solution to a stainless steel container already containing 24 ozs of distilled water, and heat the solution up close to 190 degrees fahrenheit. Add whatever steel object you want to parkerize and wait 10 ? 20 minutes, depending on the type of coating you are after. Experiment for different results.
NOTE: ALWAYS add acid to water, not water to acid, in order to prevent serious injury!
Brand Names ? approx. cost $15/gallon:
Ospho, Metal Prep, Ph-ospho-ric, Jasco Prep and Prime, Concrete Etching
Note: i used Klean Strip Phosphoric Plus. the finish came out light gery (like colt grey) then i go over it with cold blue to get the finish as dark as i need it.