It has to be stiff enough to not convex the apex, soft enough that it doesn't cause the abrasive to form deeper random scratches, and doesn't cause any scratches itself. So far I'm using veg-tanned cowhide and have yet to try balsa, but have some so it's on the to-do list. I think denim convexes the edge too much and surgical tape is too stiff. I am pretty shy of trying other fabrics for fear they will be like denim.
This is not for practical edges, looking for the absolute extreme for a BESS winning edge. I don't think the strop should have any abrasive qualities, that should be for the abrasives that are added separately.
What I have tried so far and my thoughts on it. All are mounted on EP aluminum blanks using 3M Super 77 with a finished thickness of .24".
Veg tanned cowhide, .08" thick smooth side out - My favorite but my gut is hesitant about going down to .1 micron diamond with it. My gut is wondering about balsa?
Denim, old 501s - I like how it polishes but convexes the edge noticeably more than leather.
Surgical tape - What I have is a plastic cloth and doesn't absorb the abrasive so it does not work.
Remember, I am looking for an extreme polished BESS edge so it can't convex the apex.
My short suggestion:
I would say wood or painter's tape strop with diamond abrasive medium.
I can say objectively that I am capable of producing a what can be considered a genuinely sharp edge, and that I have a high standard for my edges. However, I have not used the BESS measuring device, so I cannot tell you precisely where my results fall. My suggestions are based off the net result of a hair-whittling edge (from root to tip, so as to avoid any kind of "cheating" factor involved with the edge catching the upward-facing scales of the hair when sliced tip-to-root). The results must also be obtained quickly, a criterion at which both of my suggestions excel.
My expansion on why I suggest these two options:
The results per unit of time cost and unit of financial cost are extremely high. So high, in fact, that my ultra-fine waterstones rarely see use (as amazing as those stones are, they are very expensive for me, so I am not really eager to throw them into my daily-beater lineup).
Waxing these surfaces with a paraffin candle and then rubbing something like 60k grit diamond powder on there provides some pretty hilarious results. Paraffin wax has proved to be a surprisingly effective free-abrasive substrate.
I like wood because it doesn't even really need to be waxed: you can just cross-sand it and soak it with a non-drying oil and then it retains a hard (relative to leather), yet minutely squishy surface that holds very fine compound well. Wood and diamond pair well. I have used many woods, and I prefer poplar for its softness and because it is readily available and cheap; I source mine at Home Depot. . I then sprinkle some 60k grit diamond powder on it and go ham. You can also use ready-made diamond paste effectively on wood. As a side note, I have found the Amazon pastes to provide excellent results.
What I like about painter's tape (on a wood or glass or metal backer) is that you can control your surface-give simply by adding additional layers of the tape, allowing you to find your own personal favorite thickness. If you use chromium oxide abrasive, it draws on extremely nicely, just like a crayon, but if you want to use diamond abrasive, you do have to wax the surface with a candle before applying diamond powder.
I have not found actual diamond pastes to be good for this application as you will likely throw the tape out after only a couple uses, rendering your diamond paste consumption far too high to be economical. Other substrates also suck. I have tried diamond powder in various oils, dry, with water, petroleum jelly, and spray adhesive. None of those provided me practical or even usable results whatsoever. Paraffin wax, however, offers excellent abrasive substrate characteristics: thin, durable, it holds the compound, it doesn't smear.
I don't like any other tapes. Hockey tape is too soft and wears down very poorly, albeit actually working fine, to some degree. Other tapes suck for obvious reasons. The idea isn't to use tape, necessarily, but to use a thin substrate that holds compound well, sticks to a backer quickly and easily, and is cheap, all things that make painter's tape a perfect fit.
A diamond-paste loaded wooden strop and painter's tape loaded with green chromium oxide are both staples in my daily edge maintenance and restoration lineup.
I will also suggest the Shapton glass 30k if your pursuit of extreme sharpness alleviates you of budgetary constraints. An edge off that and cleaned on bare leather has given me exceptional results.
