Yes, Cerakote will adhere to a QPQ nitride/salt bath nitrided surface as long as the surface has been properly cleaned as is required for all Cerakote application. As was stated above, QPQ is far more wear-resistant a finish than Cerakote, by a significant margin because it is a form of case hardening. On chrome moly steel, the QPQ finish has better corrosion resistance too...but not necessarily on stainless. QPQ = Tenifer = Melonite = Tufftride... all same process.
What is misunderstood by many is the fact that the QPQ process actually REDUCES corrosion-resistance of some stainless steels, depending on chromium content and process temperature. The reason - stainless steel's corrosion-resistance is due to its higher % of chromium vs the typical alloy steels used in guns, and that higher Cr % produces a passive layer of chromium oxide on the surface. The QPQ salt-bath nitriding process binds nitrogen with iron to form iron nitride rather than iron oxide (rust). In stainless steel the introduced nitrogen also ties up the chromium at the surface to form chromium nitride, which then cannot form chromium oxide and it leaves some of the iron to oxidize. This is especially true with the 400 series stainless steels used in guns. The process still adds significant wear-resistance to stainless.
So in short, coating QPQ'd 416 SS with Cerakote does have some corrosion-resistance benefits. On QPQ'd alloy steel, Cerakote's only benefit would be to change the color from black. I think QPQ looks really nice. Its level of sheen from flat to gloss black has to do with how the surface was prepped prior to immersion in the salt bath and how much and what type of polishing was done in the 2nd stage of the process ("QPQ" = "Quench > Polish > Quench").
