The tip was to find a hidden crack, not for general etching.
I usually neutralize with TSP, rinse well under hot running water, and boil for 10-30 minutes. I dry off with clean towels, and either set the blade in a 300F oven, or give a few quick passes through the forge to get it in the 300-400F range. I them take a rag wet with light oil and work oil into and on the whole billet as it cools to room temp. I don't anything for 24 hours, and than only wipe off the excess oil. If the billet will be forged, I leave it oiled as the oil in any void may help it close up and weld shut (hydrocarbon flux weld).
Some information on this type weld:
When doing a "hot dog weld", the "U" of softer metal is usually as deep, or nearly as deep, as the insert metal thickness. This wraps the entire hard core in soft steel. The billet is closed up tight, assuring the core is seated fully into the U, and then forge welded . The weld is worked at welding heat for three or four passes to assure a good weld. Then the billet edge is cleaned up a bit and the forging to shape begins. The hard core is exposed in grinding the bevels, and will only show for the area just above the edge (about 1/4" usually). This makes a very tough blade that can take a severe impact or bend and not snap in half. The edge may crack or chip, but the knife will survive. It is a great method of making a rough use camp knife or a "wilderness and survival" short sword.
In Japanese smithing, this is called a kobuse san-mai billet ( folded three layers). If making a sword this way, the core may stick out of the bun a bit more to allow more of the hard steel ha to show for a hamon. The ji and mune are covered in soft steel skin, and will, take chops from another blade and severe abuse without the sword breaking.
