Correction pen marker fluid has a high visibility and has become a much used tool in the studio.
Originally I used the pens to delete ink lines on sketches, but now I use them when I want to mark a workpiece temporarily to designate some type of transition point where a punch mark would leave a permanent mark and a soapstone or silver pencil mark is sometimes hard to see or easily rubs off on the hot work. The white material deposited from the correction pen works something like a fast drying paint. It doesn’t come off easily even with a wire brush but can be cleanly removed with lacquer thinner.
The white line is very easy to see on hot iron. One point of caution, if the material is allowed to remain on the workpiece through several heats it may protect the underlying metal from oxidation, so the protected area might be visible on the finished piece as a local difference in reflectivity of the surface .
I find they are nice for labeling containers and for marking instructions on stock to be forged at some future time when I otherwise might forget the plan. They are handy for marking a date on machinery to record when the oil was changed for instance. In the offcut rack I sometimes record the length of the piece so I don’t have to pull out the tape each time I’m looking for a certain length.
I have tried several products and so far I prefer the Pentel brand Presto pens. I got some here on Amazon. http://www.amazon.com/gp/product/B000NNXZ4W/ref=pd_lpo_k2_dp_sr_3?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B000NNXZ4C&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=1F245Y3BJDMAZKMD6YMW