First of all, are you sure the clay worked and the spine is soft? Before I ever start polishing/etching I always see if the edge skates a file but bites into the spine.
Getting a hamon to show up with only abrasives can be a bit tricky, because you need to somehow abrade the different structures at different rates. Its more than simply sanding the softer steel faster than the hard steel, too. Want you want is an abrasive that selectively abrades different parts of the steels crystal structure differently. This gives the shimmering, ethereal cloudiness that moves as you look it from different angles. Loose fine abrasives work the best, especially if you use two different abrasives of different harnesses. Personally, I use fine crocus cloth (soft iron oxide) and superfine alumina powder from a ceramics store. You can do it with one, but it takes more time and skill.
Chemically etching is a bit easier, and you can use almost any acid that will selectively attack different steel structures at different rates. This includes citric acid (lemons, Kool-aid powder, etc), vinegar, ferric chloride and many others. The trick is to not over do it and make the etched surface so rough that you don't get any shimmer at all, just a dark vs light area.
For you, unless you have access to loose abrasives, just soak it in warm vinegar for 5-10 minutes at a time, wiping off the oxide and checking the progress until you like it. Neutralize it with ammonia or baking soda so it doesn't keep going, too. Good luck!
