I have some 185's and yes I use just a file to make the edge and then a strop to finish, cardboard will work for this.
Single bevels are not hard to sharpen. I think they are easier.
Run the file on the bevel side and then lay the "non-bevel" side on the file very lightly and remove the burr. Use progressively lighter strokes and repeat the process with the strop. Done!
I popped hair in about 60 seconds with my file and strop. create a burr, then remove it.
Bill is doing a fantastic job with these heads.
field testing begins Tuesday.