DIY toroidal transformer spot welder
A toroid core intended for AC is rarely equipped with a gap (which would
defeat the self-shielding property), so it may remain magnetized for a 
while after power is disconnected.   In the case of a spot welder, with
unknown secondary loading, saturation is highly likely to be a problem
(very high inrush current) if the AC waveform is presented (the switch
closed) at some random time.

Such a core just isn't right for this kind of many-short-bursts activation.