Any nick or scratch. It's where stresses can concentrate at a point and
cause a major failure. They can be filed out so that they don't form sharp
angles and thus redistribute the stress over a wider area.

I had a small ding on the "back" of my prop (the side you see when you're
looking at the nose from in front of the plane). During an assisted annual,
I filed it out so that it's now a smooth "dent". It has no sharp corners or
edges now.

mike regish


Incidentally, what's a "stress riser?"