The technique of TIG welding (Tungsten Inert Gas) is one whereby the 
arc is sustained between a non-consummable tungsten electrode and the 
workpiece.  Inert gas is used to prevent oxidation and subsequent 
contamination of the weld.  It is a slow technique, but because of 
that, the process lends itself to manipulation by a very good welder. 
BTW, the proper acronym should be GTAW (Gas Tungsten Arc Weld).

What Leonard is doing is to visually observe the weld VERY closely 
and watching for the corrosion and allowing it to come to the surface 
of the weld and allowing it to remain there until the weld cools.  
Then the top layer containing the contamination can be physically 
removed by machining.  NOT a technique for beginners using their own 
head.  Get a junk head and practice a few zillion times.  Preferably 
with and experienced welder who knows what to look for.

The usual body repair welding machine is a GMAW machine.  Gas Metal 
Arc Welding sustains the arc between the metal weld filler and the 
work piece so the contamination location cannot be controlled as can 
be with GTAW.

- Jeff  '80 Westi, Grass Valley, CA  (Portraying a Weld Engineer at work)
 - - - - - - - - - - - - - - 
Leonard has developed some bizzare technique whereby he floats the 
corrosion out of the pits using the TIG as he welds them up.  
Apparently he can rework very badly damaged heads.  But at a pretty 
fair price, about $250 last I checked, still a lot less than new.  
Even the guys at Stephens will steer you toward AVP for wasserboxer 
head work.