We can first assure you that Weld Through Primer isn't a gimmick and actually works! No matter the brand all weld through primers have a few things in common. First they are all mostly zinc based and when applied the zinc covers the bare metal and seals out moisture and rust.

Can you spot weld through primer?

Weld-through primer was still allowed for squeeze-type resistance spot welding, Tobie said. ... “When doing squeeze-type resistance spot welding (STRSW), apply a zinc-rich weld-through primer, or spot sealer, to bare steel areas being welded and wipe off any excess,” the Body Repair News states.

Is weld through primer rust proof?

Corrosionpedia Explains Weld Through Primer

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.