Who specifies the fabrication (or mill) size tolerances of a wire is the standard it follows, in this case a MIL one.
Now, it's perfectly possible that a certain equipment (welder or gunspray, in your case), requires a more stringent tolerance than that allowed by the standard. This is because the equipment manufacturer has decided that, for a number of reasons,
his machine requires such a stricter tolerance in order to do a good job.
My recommendation is:
1) Read the Instruction Manual of both equipment. They should state the size of the wire that is to be fed in them.
2) If this doesn't work, get in contact with both manufacturers and ask them the question. Obviuosly, they know perfectly well which size of wire their machine accepts.
Giovanni S. Crisi
Sao Paulo - Brazil