Whilst I was out in the workshop welding up a frame, I thought I'd try an experiment. My idea was to use a security marker pen, the type that only shows up under UV light. I drew a couple of lines on a scrap of steel and checked they appeared with the provided torch, the lines were visible but not particular bright.
I then proceeded to run the welding torch along the bar. The idea was that the lines should glow brightly in the UV light generated by the arc of the TIG torch.
No, nothing showed up at all.
I don't know if this is due to a poor quality pen or if the idea will not work for some scientific reason.
If you've tried this or have some pens and want to give it a go, then let me know of your results.