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I was taking a 3F test the other day on pickled and oiled 14 gauge steel and after welding the fist side of the fillet the other side got tons of mill-scale. I've tried cleaning, brushing, cooling, everything....the test turned out good. But I feel defeated that the other side is not as nice looking. I got thinking....anyone ever use brazing flux to cure this sort of thing? Thanks - RJ
Solar Flux "B" is made for that purpose, at least for stainless steel applications. (I've not tried it on mild steel but would be surprised if it didn't work simiarly.) However, it does leave behind a hard, black "glassy" residue where it got heated up, which might be objectionable. Backpurging with inert gas is another option, which is not so convenient to get set up to do, but would maintain that back side surface very nicely.
Something else you can do is setting up a chiller block (which is, just a thick, thermally conductive piece of metal). Put it on the back side of the weld can held suck the heat away from the material and keep it from oxidizing. (It will work best if you aren't trying to fully penetrate)