I have some Kobalt diagonal pliers which dulled out fast cutting plastic wire ties. I am both surprised and frustrated. My Irwin ones never dull. I read a site about using a file to sharpen diagonal cutters. However, I’ve ruined more than one blade. I’m not particularly handy and I get angled wrong, etc. Any tips? I have a rotary tool set but I believe that would be a worse mistake. Thanks in advance!