Why Were Carbines Factory Parkerized When ALL USGI Thompsons' Were Factory Blued?

[Tobor]

You can see that the majority of carbines were parked just by looking at original examples throughout carbine production. The early carbines were blued but since the finish didn't hold up well to the salt spray test a phosphate finish was adopted. The phosphate finishes didn't fare much better to the salt spray but it was an easier process. And yes there is documentation confirming the type finish applied.

BrianQ,
Could you refer me to the M1 Carbine reference where this is explained?

While there were only two entities manufacturing TSMG's during WWII, Savage and Auto Ordnance, and 10? entities manufacturing Carbines, it is odd that there wasn't a standardized finish that would be utilized for all factories for USGI small arms. Many USGI TSMG owners still believe their Parkerized Thompsons' sport the factory finish.

Phosphating is the same as Parkerizing (which was a proprietary process of the Parker Rust Proof Company just as " Du-Lite" is the name of a refinishing product) and would require that the parts be sandblasted first or the finish won't adhere. This would be a time consuming process and the parts would require protection from rust while awaiting the phosphating. There is no prep required for bluing other than the parts being clean. To machine a part, and then blue it is much faster than Parkerizing. The time savings in the middle of a war would surely tend to favor bluing, especially when the factories are already set up for bluing. The TSMG was subjected to the same government mandated proof testing, and yet they were shipped out blued.

The USGI TSMG was either blued right over the raw machined steel or sand blasted and then blued.