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Discussion Starter · #1 ·
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Noticed when I went to put my SP5 back together the other day there are some mild brownish discolorations near the back of the inner receiver and a slight one towards the bolt.

The gun is pretty new and has been meticulously maintained so I can’t imagine it’s rust or corrosion. It doesn’t wipe off so far through I’ve not tried heavy scrubbing. I contacted HK and they said it has been observed commonly on other guns and may be a cosmoline reaction that it’s normal and nothing to be concerned about.

I figured I’d ask here as you guys have been quite helpful thus far with my SP5 questions.
 

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Discussion Starter · #3 ·
I saw that. For some reason I feel that mine looks less like rust like that thread. Not sure what the discoloration is in mine, but it’s something. Does anyone notice this in their SP5?
 

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I noticed just a smidge of what I believe is surface rust on one of the inside frame rails on my SP5k. I just lubed the entire gun generously after its first cleaning.

The inside of these stamped receivers aren't finished since they are bent and welded and then finished.
 

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Discussion Starter · #5 ·
I was told they are parkerized inside and out before painted on outside so they should be protected regardless.
 

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I was told they are parkerized inside and out before painted on outside so they should be protected regardless.
Looking at mine, it looks like gray unfinished metal. I can see the black overspray of the paint on the edges of the stamped receiver but down in the inside of the gun, its all a light gray color. I'm not positive though
 

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I wouldn't worry about it. Yes, typically these HK platforms (MP5, HK91, HK93) are parkerized before being painted. The bare metal receivers are dipped into a chemical tank which reacts with the outside and inside of the receiver, thus parkerizing the metal. Parkerizing isn't really a metal protectant on its own. Its designed to absorb and hold oil on the metal, which then protects whatever part has been parkerized from corrosion. Depending on the chemicals utilized during the parkerizing process (Zinc or Manganese), it will vary in color from black to lighter gray, and even sometimes a greenish tint. I have parked lots of stuff with manganese phosphate. The inside of your receiver doesn't look like bare metal to me, but also doesn't really look like parkerizing. It looks very light, almost a whitish color, I don't know if maybe that is just the photo making it come out looking so light though.
 

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Discussion Starter · #9 ·
Well the flash was on. So in your opinion then are the brown colorations just reactions to the coatings and metals rubbing? As long as it’s not rust im fine with it.
 

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Ah yes, the flash probably makes it look lighter than it actually is. So, its probably parkerizing. I would say, yes, those brown colorations are probably some sort of staining or reaction. It does not look like rust to me. However, if park dries out too much, and its exposed to a humid environment, it will allow the metal to rust. So, you generally want to keep a healthy coating of oil on the inside of these HK sheet metal receivers. Just make sure the park on the inside is saturated with oil. You certainly don't have to go crazy and soak it down dripping everywhere or anything. Hope that helps ease your mind a bit.
 

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Discussion Starter · #11 ·
^^^^That’s helpful yes thanks I was hoping to hear that. I have oiled it lightly though perhaps I’ll add a bit more.
 
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