Powder Coating of Stainless Steel 316L

..." Surface preparation is probably the cause of the poor adhesion of the powder coating. Not all stainless steels accept phosphate pretreatments to the same extent; some being more reactive to the chemicals in the pretreatment process, and others being more inert. What you have written makes perfect sense. If you want better success with 316 stainless you might need to change the pretreatment chemicals to more aggressive reactants, or you might need to provide some physical profiling of the surface so that the powder coating can adhere chemically and physically."...

https://www.paintcenter.org/rj/jul05j.php

https://reliantfinishingsystems.com/cleaning-pretreatment-primer-part-one/

Chemical finishing and powder coating parts with installed fasteners both present problems.

The fastener must be compatible with the finishing solution. Solar Eagle's link provides some great info.

Parts with fasteners require extra rinsing and may still entrap the finishing solution between the part and the fastener. The solution may bleed out over time and cause staining or failure of a top coat. The finishing solution may remain in place and cause corrosion.

Once you have clean parts and begin to powder coat them the next problem crops up. The fasteners almost always have an air pocket around them and the outgassing of that air can cause bubbles or other issues with the powder coat.

Good luck!