Files, grinders, etc. are not the tools of a professional and neither is a nail. Each is a compromise and each lends aspects to improper blade processing and safety.
Here's the real deal:
http://www.lawnmowerforum.com/gener...ade-sharpening-pros-perspective-part-1-a.html
Here's the real deal:
http://www.lawnmowerforum.com/gener...ade-sharpening-pros-perspective-part-1-a.html