It would normally cost more than just buying another one but I already have the stuff here. I use like vinegar or an acid to remove the bluing. Lots of how-tos online. Then I have a bottle of Manganese Phosphate/Parkerizing solution here. https://www.palmettoenterprisesparkerizing.com/product-page/manganes-pint-16oz (They also sell on eBay)
I heat the solution to 200 degrees in an old coffee pot, drop my small parts in for about 20-30 minutes and done.
I have a few FSB's I removed the flashing and lug from. I have some lower parts I have "aged" too much. Small parts that I dimpled. Bolt releases that need reshaped to 601 style. I have ran across a lot of parts that needed to be manganese phosphated / parkerized so I keep the chemicals on hand. If you think you will need lots of things manganese phosphated / parkerized in the future, then the price of a bottle would pay for itself.