The ferric chloride solution will remove the oxidation, which is your brown or black color, from both the iron and the steel surfaces of your Damascus barrels. But the rust is a bit more tightly bound to the iron than the steel, so the steel portions lose color first. This is what creates the contrast we seek.
It doesn't take long at all for the ferric chloride solution to do this in the concentrations commonly recommended. The dips should be short, and you need to quickly rinse to avoid uneven etching. Experience will teach you how long, and I always advise beginners to pick up a set of junk barrels to experiment with.
Many people use a vertical PVC pipe container for their etching solution,, and this can cause problems if the concentration is on the high side. That's due to the fact that the end that goes in first is also the last to come out, so that end spends more time submerged in the ferric chloride solution. I prefer to use a weaker solution for more control, and a horizontal tank to keep things even. I store it in empty gallon jugs when I'm done, so it gets mixed the next time it is poured into my etching tank.
