So what's the purpose of this process? To remove any residual carbon or other stains from the weld for appearance sake?
It is primarily appearance, it's stainless, too obvious as it won't be painted. There is mostly the rainbow colored oxidation. Can't say I've noticed carbon soot before, video looks standard speed, yet much faster dissolving too. When it pooled and dried, had the appearance of baking soda. The battery cart might be visible upper right.
...we'll learn more by wandering than searching...