The inspector and the contractor should be aware that the initial condition of the steel can vary within a short area.

The initial condition is deteriorated paint.

Example of Clean to Bare Substrate and Very Thorough Cleaning with discoloration on right side where the coating was not intact of adhered to the substrate.

There is a very slight yellow hue to the steel on the upper left that is the onset of light flash rust.  The condition of the steel under the paint is a mix of Grade D and Grade C.

If the coating is intact, the steel will look close to the original condition.  If there is corroded or pitted steel, there often is a very-dark discoloration – gray/brown to black- that cannot be removed by further WJ.

The coatings manufacturers have said that “Analysis shows that this thin film consists mainly of ferric oxide which is an inert material. Because it is tightly adherent, it does not present a serious contamination problem.”

Author’s note- This discoloration has been over coated since 1985 with no apparent effect on subsequent paint performance.

Distance from the surface will affect your judgment.

If you’re too far away, you won’t be able to see through the flash rust in order to determine the difference between “steel substrate may be observed” [Light flash rust] and “obscures the original steel surface” [Medium flash rust].
Heavy flash rust simply “hides the initial surface condition completely.”

For the purposes of training, and consistent quality, the Todd Pacific standard default rule is: To properly evaluate and inspect waterjetted surfaces with flash rust, the Inspector will be within touching [by hand] distance and no more than an arm’s reach from the surface being evaluated; the same as the coating’s applicator.