Parkerizing isn't a coating, as previously stated. It is a crystalline structure bonded to the surface of steel which provides an excellent surface media to hold oil, grease, etc. Parker does little to prevent rust, but the oil, grease, or other coatings which it holds provide the rust inhibiting quality. Parkerized surfaces absent oil, grease, or coatings such as Dura Coat will rust. The greenish patina found on military weapons is a result of many years of aging of oils and greases used on parkerized surfaces and is difficult to duplicate.
Parkerizing is not necessary to form a durable bond between metal and coatings. Thorough de-greasing followed by Sand blasting with 120 grit aluminum oxide and immediately followed by whatever coating you choose will give good results. I have many firearms which have been Dura Coated several years ago and they have shown very acceptable resistance to weather and wear. Dura Coat also sells a product call Dura Heat which is similar, if not the same as CeraKoat. It is air cured, ceramic based and extremely tough. Light coats are the key, as it has more of a tendency to run than std DuraCoat, and is less expensive than CeraKoat.
We just got a class 101 in coating, I made my job alot harder than I should have I'm glad I wont follow your advise when doing customers guns, been blueing, parkerizing, nickle plating, powder coat, crome and coating since 1980. If parkerzing doesn't offer anything to pervent rust why has my towing hitch hasn't rusted since it's been on my truck since 2004 and no I don't oil it either. Yes it hold oil but it also helps to prevent rust. Parkerizing isn't necessary to coat but last alot longer if done. You can'tparkerize stain so sand blasting is best but for crome moly steel I always parkerize