I don't use a leather strop or cardboard, nor a ceramic, nor a diamond hone.
I hollow grind mine lightly on a belt sander with 80grit, then finish with a small bastard file. The key is med. to strong pressure at first and lighten up to just barely touching toward the end. I can have a brand new one shaving sharp in less than 5 min.
It took me awhile to learn the proper pressure and technique, but now it's not a problem.
On occasion I do run into one that seems like it has a super hard spot in the middle of one or two blades that the file just seems so slide over without cutting and I have to touch it on the sanding belt again and start over.
I think this is due to uneven tempering when the heads are hardened?