I tried it once just for fun a few years ago. I can't really remember the exact setup but I think I used eight 250w panels in 2s4p configuration. I think I was able to light up a 3.2mm (1/8") rod but not well, and it ran a 2.5mm rod quite well. I think putting all 8 panels in parallel would have been better but I didn't want to use up a lot of MC4 connectors and cable to do it.
Its not really practical, for me anyway, its just too easy to plug my normal welders in and weld anything anywhere within reach of a 40m extension lead.
But I can imagine in some sort of post apocalyptic Mad Max scenario you could set up a solar powered welding shop. It should be possible to control the current quite well just by adjusting how many panels you have in parallel.