MP gives a darker, sometimes 'near black' color and is generally considered to be more durable than ZP. The U.S. adapted MP toward the end of WWII.

 

To add what 1saxman said. MP has a larger crystalline structure that is coarser than ZP. MP solution can be saved and reused, but ZP is not supposed to be reused.
MP supposedly can be used as a basis for paint, due to its grainier structure.

 

WW2-era weapons were usually zinc phosphate, except for the small parts which were often manganese due to the hardened small parts not taking the zinc very well. Post-war nearly everything was manganese.

 

The original Parker process (Parkerizing) was originally developed as an undercoat for paint.

 

The primary reason for its use on weapons is its oil holding properties.

 

When I handled my friends parked Colt Comp it was chalky, I oiled it up, let it absorb, then wiped it down. He was amazed at the difference. Satin smooth! A well oiled parkerized finish has a very unique feel. My parked Tisas USAF .45 is an amazing parked finish, better than Colts in my opinion, and I lightly use an oil damped lint free rag to wipe it down after every use.

 

Interesting stuff guys! I can see it working well as a primer, and we know it holds oil pretty well.

 

Holds Cerakote incredibly well. Match made in Heaven.