Figure 1	Figure 2	Figure 3	Figure 4

STRSW process
    To understand how shunting works and why it is sometimes necessary, consider how a resistance spot weld is made. A short burst of current flows between the electrode tips when the spot weld trigger is pressed. The nature of current is to flow through the path of least resistance. If there is a conductive surface between the copper-based electrode tips, such as bare steel, that is the path of least resistance. But since steel is not as good of a conductor as the copper electrode tips, there is resistance to the current flow, enough resistance to heat the steel to a molten state. Pressure applied to the electrode tips both before and after the short burst of current helps contain the molten steel to that spot (see Figure 1).
Shunting coated steel
    Conductive coatings on the steel, such as zinc or weld-through primer, increase the resistance but still allow a spot weld to be made. Nonconductive coatings do not allow a weld to be made. The most common of these is E-coat, the factory corrosion-resistant primer applied to every original and replacement body panel.
    Most of the E-coat on the mating surfaces can be preserved when replacing a part with STRSW through the use of a shunting clamp. The E-coat only has to be removed on the outside surfaces. The shunting clamp is positioned at the first weld site and the first spot weld is made at the adjacent site.
    When the spot weld trigger is pressed, current flows from one electrode tip. When the current meets the nonconductive coating at the mating surface, it looks for the nearest conductive path which is through the top workpiece, around the shunting clamp, and back through the other electrode tip. The E-coat at the mating surfaces burns away. This is all happening very quickly. In fact, the majority of the current, in the short burst of current, travels between the electrode tips. The detour through the shunting clamp was only a brief, but necessary, diversion (see Figure 2).
    Note that the E-coat does not have to be removed between the spot weld sites on the exterior of the flange. The current traveling through the coated workpiece is like current traveling through a wire coated with insulation.
    Successive welds should not require a shunt. The previous spot weld serves as the initial conductive path. In fact, any other clamps used for joint fit-up should have the jaws wrapped in tape for insulation (see Figure 3). A shunt is again required when another series of spot welds are started.