I can’t give any definite answers. I have had occasional spots (steaks is more accurate in our case, similar to what your part edges look like) that didn’t dye well, which we have found was from inadequate degreasing.
In the upper right corner of your picture, I can see the effects of stripping. There are what appear to be shallow, flat-bottomed valleys.
The light colored pin spots look like ones I have seen with 2000 series alloys, both from commercial places and ones we have done. It happens (to us at least) when multiple pieces of 2011 series are racked together, and dyed for extended periods. I have never seen it happen when individual pieces are dyed alone, or when multiple parts are done at the same time on separate hanging wires.
Maybe your part is a 2000 series alloy, and maybe not. Here is some information that might help you to determine if it is a 2000 series alloy or not. When 2000 series is dipped in a caustic, either for cleaning, stripping, or pre-anodize etching, it will build a layer of smut at a much quicker rate than 6061. We have manufactured identical parts in both alloys, and occasional mixed 1 or 2 together by accident. After a couple of minutes in hot cleaner, the 2000 series parts stick out like a sore thumb because they get a dark stain. The same thing happens in the stripper or etching solution, except it is a smut that will wipe off. We use sodium hydroxide for both etching and stripping, but have separate tanks for each purpose. So, if you have some known alloys on hand, you might compare the reaction compared to your trouble part. While I am on the subject, 6063 is significantly more resistant to getting smutty than 6061, or any other alloys we have experience with.
I have a hunch that your problems on the edges could be dissolution. Is it lighter there right after dying, or does it show up more after sealing?
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