Ferric Chloride, sold at Radio Shack as an etchant for electronic boards is commonly used as an etchant for knife blades. I dilute mine about 4 parts water to 1 part Ferric Chloride. Also it etches faster if the solution is heated to about 150 degrees or so.
For a mask coat you can use fingernail polish. When dry scribe your line in it. After etching remove the fingernail polish with acetone. I use Red nail polish because it is easy to see and easy to make sure you have all of it removed.
I also have a written procedure for etching using a salt solution and an electrical current, works well for small stuff but remember that introducing an electrical current into salt water produces chlorine gas. You have to make your own stencil though but that can be done with scotch tape.