Selecting rectifiers for a welding machine
You may want to put so-called "flyback diode" or "snubber diode" paralel to L2 to prevent back-EMF spikes. High-voltage arc starter should be connected by means of transformer put in series with welding leads, not in parallel. Bypass cap won't do the trick, no matter where you want to put - it will have infinite impedance for the DC, so you can't put it in series with L2 and will produce tiny impedance (and thus no effect) if you put it in series with your HF source. Take a look at this link for some clarifications. Another thing you can do to make arc start easier is to put electrolytic cap (mind the polarity and voltage) at the rectifier output (but it should be connected before L2). 4700uF will be sufficient capacitance value, but it may reduce your welder power factor.