Water quality can make a difference but in my rather large experience in rust bluing, the most common cause of spotting is an unclean surface. Sometimes castings can be very difficult to color evenly, something that is rarely a problem with parts made from bar stock, and sometimes there are microscopic pits and cracks in pieces that hold oil which creeps out on the surface after you degrease and gives yellow spots. I recolored a 1800 vintage English damascus rifle barrel that spotted terribly the first time around. Repolished it and soaked it in acetone for a couple of days to get the oil out of those tiny cracks and pits afterwhich it colored fine.
Tom
