Both orders obey one rule: the particles must be free to move while a field is present. The only difference is whether that field is still being applied, or is the one the part kept.
Why dip instead of pour? With the current off, only the retained field holds the particles — so the bath is applied gently, by immersion or a low-velocity flood. Indications build while the part is in the bath, and the drain clears the background off the rest of the surface.
Lift it out slowly. The classic residual mistake is pulling the part out fast — on a smooth surface that strips the indication straight back off.
Only on high-retentivity steel, and less sensitive than continuous: the retained field is weaker than an applied one, so fine indications can be lost. A soft, high-permeability part keeps almost no leakage field once the current stops.
Why only one shot here? E1444’s “at least ½ second, minimum two shots” rule is written for the wet continuous application — the second shot exists to build indications after the suspension flow has been interrupted. In residual what matters is that the shot leaves an adequate retained field, proven on reference parts with QQIs, not by a formula.
per ASTM E1444 / E709 — verify against the edition and spec your shop invokes
Mild (low-carbon) steel is soft — low retentivity. It loses almost all of its field the instant the current stops, so there’s no residual field left to gather particles.
The residual technique needs a hardened, high-retentivity part. Pick the tool-steel part.
The opposite order from continuous — shoot → stop → dip. Magnetize the part, switch the current off, and then put it in the bath; the field the part keeps does the work.