I used the same sample piece of copper for etching with ferric chloride since it was still masked with duct tape from Etching Round 1. I traced over the monograms with a Sharpie oil-based paint marker as the resist, which seems to have a more consistent application than a regular Sharpie but is much easier to use than nail polish. (Available in three thicknesses where art supplies are sold, in my case at Michaels.) I let the copper plate sit in the etching solution for about an hour, checking it every 20 minutes or so to see how it was going. It's hard for me to tell how much etching is happening because the resist itself is three-dimensional, but finally I couldn't wait any longer so I rinsed the piece off and removed the resist with acetone.
You can actually see both rounds of etching in the top of the closer monogram and the circles around both (which were only salt water etched). Etching is a much more subtle texture than what I normally do with PMC. And, like I said, it's hard to tell how much etching is happening during the process, so I guess salt water etching was more effective than I thought.
At least now I can honestly say I can etch copper.