if it is true Damascus and not the newer lookalike stuff, after u sand the blade 600 to 800 finish then you can try vinegar and lemon juice soak, 2hrs, check etch. lightly rub it on some 2k w/d sandpaper. if not dark enough, repeat. sometimes it take two or three times to bring it dark enough sometimes one will do it. ferric chloride is another ballgame, I warn you is messy stuff, any metal it touches it will discolor, and I literally mean any metal. this stuff is for etching pc boards in electronics and will attack and eat copper. the lemon juice 25% and 75% vinegar is a better and more environmentally friendly approach. the Damascus I make is 1n520,52100, 1095 mix and always etches well with this method. I do use a bubble chamber too, nothing real fancy it is a 4 inch pvc capped pipe at one end with a hole drilled in the cap and aquarium airline tubing sealed with silicone and a airstone inside the tube with a small air pump to keep the solution agitated. I have left knife blades submerged for 6 hrs with no real damage. either way you go the edge will have to be redone after it is re-etched.now if it is only acid etched Damascus lookalike you can do nothing as these have a laser type or sandblasted etch, they are not folded steel.