Good morning, Dan. I won't speak to the details of exactly what's going on (whether the acidity of coffee is actually etching, or if the coffee is more of a stain, which is what I believe), but I have been using the same 1/2 gallon of strong, cheap, instant coffee on my damascus blades for the past 4+ years, and here's my process based on my experience.
First I do my normal ferric chloride etch (three 30 minute cycles, cleaning off the oxides with 0000 steel wool after the 1st and 2nd cycles) to bring out some topography of the pattern. After the 3rd, I neutralize the blade in boiling water with baking soda in it. I then take the blade out and let it air dry, usually overnight. (I'm not sure it's necessary to let it sit that long, but I seem to have better outcomes when letting it sit longer than shorter.) Then I use 5000 grit wet/dry sandpaper wrapped TIGHTLY around a sanding stick and polish the black oxides off the high spots (the nickel steel that resists the etching from the ferric chloride). If everything looks good, I'll leave it there. But I've found that sometimes (usually on lower layer patterns or san-mai blades) it's hard to avoid cleaning the oxides off the 1080, and then I'll finish up with a coffee "etch" or dunk (here's where the science turns to art....you'll have to play around with what's enough and what's too much). I start with an hour, then do more if necessary. I usually have to clean off the nickle steel with the 5000 grit paper a little because the coffee tends to "stain" this too.
IMO, the coffee is good because you can re-do this many times whereas if one used ferric chloride, the plain carbon steels would be etched away ruining the blade.
I hope this makes sense. If not, feel free to ask.
