[timshufflin]

Lot's of things. One of them I see commonly is on reproduction parts. Examples I see are on reproduction Garand stock metal including the butt plate, sling swivel, stock ferule. On 1911's some of the frames made in South America take park badly, they actually show the exoskeleton of the frame. I'm guessing that the metal used in these examples is akin to some sort of pot metal.

I purchased a Coonan receiver for my FAL and I had these streaks on the outside of the mag well. I ended up having to sand down my mag well with AL oxide sand paper to get what I could only call "impurities" out of the metal and then go again, it worked. There seemed to be some sort of foreign metal in the casting, maybe even sand. You could actually see swirling in the metal when you media blasted it, you knew it wasn't going to park right and it didn't.

You said streaks and not dots so these are the most common things I've found. These are assuming that the prep work is done right.

 

[north cal]

Streaks in parkerizing

Thank you Tim, the streaks I am talking about are on a m1a rifle I had reparked. The barrel turned out buitifull. But the receiver has a couple of black streaks by the heal stamp. They can only be seen if you turn the rifle just right in the lighting. And I can not capture them in a picture with my tablet. So again thank you very much for your reply.

 

[timshufflin]

Thank you Tim, the streaks I am talking about are on a m1a rifle I had reparked. The barrel turned out buitifull. But the receiver has a couple of black streaks by the heal stamp. They can only be seen if you turn the rifle just right in the lighting. And I can not capture them in a picture with my tablet. So again thank you very much for your reply.

You are welcome Sir. I'm guessing, it's an M1A, cast receiver then. Metal does funny things, there are even roll marked receivers out there where you can see the outer markings from the rolling device after a park job. It seems that zinc is the least finicky park for taking over metal with "questionable" metal make up.