Rust spots on nitric acid passivated stainless steel

First be sure the organic soil is completely removed and not re-deposited as floating in the rinse. Then be sure the passivation solution is below 2%/weight dissolved iron. Then be sure the particular passivation solution is approved for the alloy that is rusting.

Ryan,
It sounds like you are using a Type 3(A967)/Type VII(QQ-P-35) nitric bath, but you didn't mention the temperature or process time used, or the exact grade(s) of stainless involved. Assuming your passivation bath is being run properly, you may also be getting surface iron contamination on your parts, either too much for the passivation process you are using to fully remove or it is occurring after passivation. Also, what type of environment are the parts being installed in? A highly corrosive atmosphere will eventually overcome any passivation, though a better passivation will hold out for longer. Let us know if we can help.

If there are hundreds of tiny rust spots, It quite probably was because the customer ran the parts on a rotary brush that was made of 410SS. There is enough iron in 410 to rust. If there are tiny pieces of 410 stuck in the metal, in my experience, there is no amount of cleaning / passivization that will allow it to pass the salt spray test.

Another problem is the customer uses the same blast cabinet on the SS as they use on steel parts.

It takes a lot of talking to convince a customer that it is NOT OK to use 410 on any SS. The standard reply is we do not do that or "but it is SS".

Do a search on this forum, this question comes up again and again.

The most likely cause is iron contaminated shot/grain media before passivation. The easy way to check is to run a (covered) magnet over it.