We've done it both on 1050 and stainless but I can't recall if the etched design actually crossed any of the weld lines. I suspect you might get a slightly different depth of etch in the weld area do to the different micro-structure.
I have and it brought out the welds as a different color and texture. I suspect it was the alloy of rod I was using as opposed to the sheet. This was with hot rolled that had been de scaled already. The etch took beautifully and deep, but the weld looked like silver sharpie marks. I hit it with scotchbrite and it all blended back together.
Oil-based spray paint worked fine as a blanket resist for me, and I detailed with a fine brush and some rustolium I had laying around. If your needs are more detailed than that... can't help ya.
Memento, homo, quod cinis es! Et in cenerem reverentis!
