What Does Weld Through Primer Mean?
A weld through primer is a specially designed primer applied on welded surfaces. These primers are generally applied prior to the welding process. The purpose of a weld through primer is to prevent corrosion and rusting of metal joints where two metal pieces are welded together and provide permanent corrosion protection on the inner edges of the welded metal pieces.
Corrosionpedia Explains Weld Through Primer
When metal pieces are welded together there are portions (e.g., plug welds, lap joints) that become inaccessible for a proper application of primer, coating or surface finishing. The solution for these portions is to spray a weldable primer over these surfaces, perform the welding and then spray primer over the completed weld. This is completely rust proof because the weld pool will burn off a bit of protective paint around the weld. Seam sealer paint is applied to rust proof a lap joint.
In many cases zinc primer is applied on both faces of the plug weld, then seam sealer is applied because it is easier to smooth into seams instead of the alternative, which is to smooth the seam by hand wearing gloves wetted with panel wipe, stone chipping the coated area, painting a top coat and finally applying wax protection for permanently rust proofing the weld.