Parkerizing by itself is nothing more than a very thin layer of crystalized material. If you looked at it under a microscope, it would look similar to a sponge but with sharp edges. It has no durabilty by itself. It get's it's durabilty because like a sponge, it will absorb liquids and hold those liquids in the "pores". The idea behind Parkerizing is for it to be submerged in oil while it's still "fresh". Some prefer hot oil, others cold oil. Once that oil is in the "pores", it's pretty difficult to get it all out again. (See note below)
The reason that many firearms refinishers prefer to use Parkerizing under the coating, is because the "pores" will absorb the coating too. It gives a much better bond to the top finish. If Parkerizing earns a #7 and a quality resin coating earns a #7, the two together earn a #20.
: Once the "pores" have been saturated with oil, it's almost impossible to get it all out. This is why quality refinishers will refuse to apply any type of coating over "old" Parkerizing. No matter how well it's cleaned, the risk of contaminating the new finish is to high.
There are two types of Parkerizing used on firearms: Zinc based and Magnesium based. Magnesium based results in finer "pores" and a darker color. Zinc based results in coarser "pores" and a lighter color. Most of the old military stuff was Zinc based. I'm not sure of the exact reason but maybe because of the coarser "pores" holding the oil better or more oil? We use the Zinc based Parkerizing under the resin because of the coarser "pores". It soaks up the resin much better.
Now, a few words about coatings: "Bullet proof, will never wear, just spray it on and it's done, it'll last forever no matter what you do to or with the gun, etc" is all B.S.
!! I have been applying firearms finishes for 35 years and there is no finish that will not show some type of wear eventually. My favorite statement is: "Just degrease it and spray it on". Your firearms are expensive! If you're going to pay somebody to apply a new finish to them, ask first what you're getting. Will the internals be coated too? Is it rustproof? Says who? This MilSpec rating...what's it for? Will you warranty it? Parkerizing by itself is pretty hard to mess up as long as the solution is seasoned and the prep work is good but: If you're getting a coating applied over that, ask questions!! Keep yer powder dry, Mac.
Tuff-Gun Finishes. The Name Says It All.
Mac's Shootin' Irons