How do I identify a bad sharpener?
All sharpeners remove steel. When sharpening a knife, you ideally want to remove as little steel as possible to get a very specific angle on the blade edge.

If you were getting a blade sharpened professionally, they would typically go through different grits to get it smoother and smoother and eventually a polished edge. For a home pull through sharpener, you're using a single grit that will do most of the work. Ideally it would be something in the middle that will maybe take a little longer to remove nicks in your blade &amp; grind the edge down, but not pull off too much steel.

There are two dangers with a cheap sharpener, that I can think of:


It removes too much. If the grit is too rough, it'll burn through your knife, destroying your blade quicker than necessary.
It sharpens unevenly/angle is wrong. If the angle is wrong, it could either make the blade more prone to chipping by making it too narrow, or not sharp by making it too wide, thus wasting your time. If it's really bad, I guess it could produce an uneven edge too.


Hard to say what is exactly happening in your case. That all said, if it's a cheap blade and your knife is sharper, then use it maybe? Without knowing what it was, I don't know that I'd gamble with a good blade. "Cheap plastic board with a in-built sharpener" is probably a clue that it's cheap though.