The Resistance Welding Manufacturers Association has the "Bible" on spot welding. You can also get some of the basic info from the electrode distributor's catalogs or their customer service/technical service departments.
My preferred contacts are with Taylor Winfield/Portage Transformer (OH) and American Industrial Equipment(NJ).
I was introduced to spot welding 8 1/2 years ago and these two companies gave me the best and friendliest technical assistance (and still do). I did query most of the biggest manufacturers.
