Another question. These circuit boards were originally sprayed with
some coating. I surmise that the purpose of it is to prevent
accidental shorts due to dust contamination or other debris being
drawn into the unit.

This was, in fact, a great thing, as this particular welder had so
much dust inside, that I was shocked. The amount of dust was nothing
short of incredible, maybe 5 lbs of dust settled on absolutely
everything. I took this welder out of its protective shell, vacuumed
it, and then blew the dust out using compressed air (outdoors). The
truly amazing thing, is that it was still functioning fine with all
that dust inside.

Thanks to the Miller company.

Having said that, I would like to apply that coating to the areas that
I re-soldered. What is that coating?

i