There's only one way to successfully re-sharpen a knife blade. You have to use something abrasive to remove an extremely small amount of steel from the edge of your knife and reestablish the burr. You won't wear down your blade if you sharpen a knife only when necessary.

Can some knives not be sharpened?

Bad knife steel

Unsuitable steel is the most common reason why knives cannot be sharpened. ... Such knives can then either not get really sharp at all and additionally become dull after a short time. This means that not only do you not get these knives really sharp, but you also have to sharpen them far too often.

What kind of knives can be sharpened?

Yes, you can effectively sharpen your chef's knife or paring knife at home, but serrated and scalloped knives tend to be a bit trickier. Consider taking these knives to a professional to sharpen.