[charlie]

in general, anything that is on top will affect the test in some way. The only way to be 100% sure if you coat a test block the same way and document the results.

You also have to take into account what kind of phosphate coating it is, because there are several with different “base materials”. It will also affect the electrical resistance and therefore your magnetic field in the part (keep demag in mind).

Another thing to keep in mind is what kind of defects you are looking for. Big cracks from heat treatment (gears etc.) or really fine cracks after use (springs etc.).

The surface of the coating can also mess with your powder, since its a bit porous – so you might want to use a different powder (bigger grain size) to avoid background glow.