stropping compounds – Peachey Conservation

Some of the stropping sprays, pastes, and substrates I’ve been experimenting with.

A meter was originally intended to be one ten-millionth of the distance from the North Pole to the South Pole where it passed through Paris. The Measure of All Things is a facinating book by Ken Adler which documents this feat of triangulation — in the middle of the French revolution, no less — and also explores how the defined length of a meter has since changed. A millionth of a meter is a micron. As a point of reference, a hair on your head is about 40 microns wide. A thousandth of a micron is a nanometer. Yes, I’ve been thinking small!

Typically, I hand sharpen following a grit progression of 80, 40, 15, 5, micron on 3M microfinishng films with water as a lubricant, strop with a .5 micron Chromium Oxide (CrO2), honing compound on the flesh side of a horse butt strop, then finish stropping on naked flesh side kangaroo. Don’t get me wrong, this works quite well. And there are many other ways to sharpen a knife.

Inspired by some other sharpening approaches, two aspects of my routine seemed to need a little tweaking. First, I eliminated the large jump between 5 and .5 micron, and found some finer grits for a final stropping. Adding a 1 and .3 micron 3M PSA finishing film filled in the gap nicely during sharpening. And a final stropping with a .1 micron Poly Crystalline Diamond (PCD) diamond on polyester leather has dialed up the sharpness to eleven.

3M finishing film. The lime green is one micron, and the very bluish looking (in this image) white is .3 micron. The delrin plate is in the back.

PCD or Cubic Boron Nitride (CBN) compounds smaller than .25 micron don’t work well on real leather for two reasons: the expensive spray soaks into the leather and disappears alarmingly fast, and the natural abrasiveness of the leather itself is sometimes coarser than the spray.

One solution is to use a polyester leather, which is similar to “nanocloth”, a term Ken Swartz has coined and a great product he sells. Polyester leather is made from an ultra micro fiber that holds sub-micron sprays incredibly well, is very thin so the cutting edge does not become rounded, and is extraordinarily durable. Human hair is roughly 20 denier, but this ultra micro fiber is .04 denier. Denier is the mass in grams of 9000 meters of a given fiber. It is difficult to imagine how small and light this fiber is: 9 kilometers (over 5.5 miles) of it only weights .04 grams! All of these tiny little fibers hold the diamond particles loosely while allowing them to move around a bit, exposing new sharp edges. I think this is why they last so long.

In other words, this polyester leather is a perfect substrate for .25 micron and smaller sized sprays. I’ve experimented with the .25 micron (~64,000 grit, 250 nanometer), .1 micron ( ~160,000 grit, 100 nanometer), and .025 micron ( ~640,000 grit, 25 nanometer). These are available in PCD and CBN. The diamond seems to stay sharp longer (because of the shape and hardness?), cuts a bit faster, though is more expensive. The .25 micron is pretty close to the .5 micron CrO2 I usually use, and though it does cut quicker and lasts longer, it seems an unnecessary expense. Waxy pastes don’t apply or stick well to polyester leather.

In terms of initial cutting performance and cutting edge longevity, I can’t really tell much, if any, difference between blades stropped with the .1 micron or .025 micron. Even so, the idea of a one fortieth of a micron edge does have an almost irrational appeal, but is it just a placebo effect? Also theoretically, the smaller the grit progression in your sharpening sequence, the finer the cutting edge, and the faster you get there. But everyone has to decide for themselves if the trade off in time spent sharpening is worth the final result.

Diamond compounds are expensive, but once they are loaded onto the polyester leather they last for a long time. In my experiments, I’ve used a single polyester leather strop loaded with .1 micron for over 100 knives without recharging, and it isn’t dead yet.

I’m a convert to this new sequence. It really doesn’t take much additional time, and the resulting edge is better. All the knives I make now follow a 80, 40, 15, 5, 1, .3 micron sharpening sequence, and a .1 micron stropping. When I am paring leather for my own projects, I do a two stage stropping sequence to keep the knife sharp. First, a .5 micron CrO2 on horse butt followed by .1 micron PCD on polyester leather. Once the edge becomes too obtuse, then it is time to resharpen.

Choose your poison and treat yourself to a sharpest knife you’ve ever experienced for this Christmas!

SUPERSTROP

The Superstrop has a half inch thick cast acrylic core, which is the flattest plastic available, as well as being very dimensionally stable. Flesh side horse butt is mounted on one side and flesh side polyester ultra-microfiber leather on the other. The strop has a nice heft, about 14 ounces, so it doesn’t move around on the bench while stropping. The polyester leather comes loaded with .1 micron Poly Crystalline Diamond (PCD) compound, which should last a very long time. Sub-micron diamond replacement sprays are readily available. Replacement PSA horse butt and PSA Polyester leather is also available. When working, I like to use the .5 micron Chromium Oxide (CrO2) honing compound on the horse butt, wipe off the knife to prevent grit contamination, then finish with the .1 micron PCD. Also available with polyester leather on both sides, loaded with .1 and .025 micron PCD.

SuperStrop. 14″ x 2.5″ x ~.625″. $85.00

Replacement ~15″ x 3″ PSA flesh side horsebutt: $35.00

Replacement ~15″ x 3″ PSA flesh side polyester leather: $35.00

3M PSA FILM, ONE AND .3 MICRON.

3M finishing films.1 micron is lime green and .3 micron is white.

Delrin plate, machined and lapped flat. Fits into my sharpening system. 12 x 2 x .5″: $50.00

1 micron and .3 micron 3M PSA finishing film, 4 sheets each. 12 x 2″: $10.00

The following is a reposting of Derek Cohen’s “Stropping with green compound verses diamond paste”, which originally appeared on his website, In The Woodshop in January 2009. I thought it was quite well done (and not just because he likes my horsebutt strop) and since I often get questions about how to apply the compound, or what compound to use, he kindly granted permission for me to repost. I collected some of my own thoughts on stropping here. Although he discusses a woodworkers chisel, the basic principals are the same for paring knives, spokeshave blades, etc. He has a number of other tutorials about sharpening on his website. At the bottom of this post is an update after using the horsebutt strop for five years.

COMPARISON OF GREEN COMPOUND AND DIAMOND PASTES

I have been using a horsebutt strop with green buffing compound (.5 microns) for about two years. The strop is excellent with a hard and resilient surface, but I would recommend glueing it to hardwood for certain flatness. Used on their own they have a tendency to curl slightly, and this can lead to dubbing if one is not careful.

With the green compound I add a dribble of baby oil (which is just scented mineral oil). This turns the wax into a soft paste and allows the leather to soak it up. Used as a “crayon” alone it can become thick and collect on the surface (although it must be noted that the act of dragging a blade across this will remove – scrape away – the excess wax). Lee Valley also notes, “Frequent, light applications are better than less frequent, heavy applications”.

This is my strop with green compound applied.

For the purposes of comparison I set up another (unused) horse butt strop. There were two points of departure, but I did not think that these would interfere with the results. The first was that I used the rough (flesh) side of the strop since I wanted to save the smooth (hair) side for future use if this experiment failed. And second, I did not glue the strop to hardwood (but instead held it flat on ply) for the same reason.

To prepare the strop for the diamond paste I first moistened the surface with the baby oil.

The diamond paste was part of a batch I bought on eBay about 2 years ago (.5, 1, 10 and 40 microns). This is oil-based.

LV (Lee Valley) and TFWW (Tools for Working Wood) both sell water-based diamond paste. I generally use waterstones and, that this is oil-based, is not a concern since the two mediums are not used together.

So, dribble a little diamond paste onto the strop…

… and massage it in …

One observation about paste is that the excess sits on the surface of the strop in exactly the same way as the green paste. I would say that the criticism levied at the green rouge should equally apply to other pastes. Still, as I pointed out earlier, the excess is scraped off as one drags the blade across the leather.

I used 1/2″ and 3/4″ Blue Spruce chisels on Radiata Pine end grain. The chisels could cut but really needed work.

The chisel was stropped both back and bevel. Note that this involved drawing the blade towards oneself, with the sharp end trailing. Otherwise the blade will slice up the leather.

The result – the chisel was now several orders of magnitude sharper and had no difficulty taking fine shavings in this horrible wood:

Below is the result with the diamond paste – the chisel was now capable of excellent work:

Having gone back-and-forth between the two pastes, I concluded that they appeared to produce a cut that was almost identical. With he green compound, however, the chisel did appear to cut a tad more easily and leave a slightly smoother surface. When stropping it felt smooth, while the diamond paste felt “gritty” – but this may have been due to the rougher leather (still, it was not a “roughness” that I experienced).

Bottom line – I think that either system would work well. I shall continue using the diamond paste strop as I think that this should improve as the leather builds up more diamond within it.

USING A STROP

For those new to this, I have a few images of the stropping technique I use. Others may do this differently and, if so, I hope that you will post here.

Stropping the back of a chisel (or substitute a BD [Bevel Down] plane blade here):

Note that the blade is angled at 45 degrees, held flat, and pulled towards oneself (i.e. sharp end trailing). Do not push the blade bevel first – it will slice up the leather.

This chisel, like my BD blades, is hollow ground. Find the point of contact, then drag the blade back (as outline above).

Stropping a BU blade:

My BU [Bevel Up] blades typically have a 25 degree primary bevel and a high secondary/micro bevel (e.g. 50 degrees, to create a 62 degree included angle). The microbevel makes it difficult, if not impossible, to freehand strop accurately (that is, maintaining the secondary bevel angle). Consequently I only strop the back of the blade. This works well enough to give the blade a second lease on life.

Update: It is now 5 years on, and I continue to use the same strop with the green compound. It looks the same – the horse butt leather has lasted extremely well. What of the diamond paste? Well I did not like the feel – the subjective sense of grittiness remained. I learned that others experienced something similar. The strop with green compound is less used since there is a sharpening bench alongside the work bench, and it is just as easy to use a 13000 Sigma ceramic waterstone, and in some cases this is better since there is no chance of dubbing the edge. I have continued to use the strop with green compound to “refresh” mortice chisels since these are honed with a rounded bevel.

I glued the undressed strop was to hardwood with the smooth (hair) side facing up. I have kept it that way, and it is used after honing blades to ensure that the wire edge on plane and chisel blades has been removed.

April 2014