The PUK is a term for a welder like the PUK 2 welder. It is simply
a tack welder. Concerning the white metal because of its high
reflectivity of the arc you need to darken the area that is to be
welded, usually with a marks a lot pen.
A PUK is a brand of pulse arc welder, it is a Tungsten Inert Gas
(TIG) welding tool. It is different from a tack welder in that it
welds by striking an arc between a tungsten electrode and the work to
melt a weld pool. A tack welder strikes the arc between two pieces of
the work item like say an ear post and earring.
There is no need to darken the metal, you must be thinking about a
laser where that is sometimes needed. Anything used to darken the
metal would just contaminate the weld.
James Binnion Metal Arts