Lets say I sharpen my knife at 20 degrees, at what angle do I strop at?
short answer is 18 degrees, 2 less the the sharpening angle. Use light to medium pressure to avoid the flexible leather rounding off and dulling the apex as it passes by. Happy sharpening!
Is that because the leather has more ‘give’ in it than a solid stone?
At that angle are you actually stropping the edge or the bevel?
Just trying to work out the physics behind it. At the moment I’m stropping at the same angle I’m sharpening at.
Yes, that’s because leather has more “give” to it. If you use a balsa strop and don’t use too much pressure, there’s not need to lower the angle.
My .02, experiment and see what works best for you.
Bill: I’ve never used balsa, but FWIW I never used any wetting agent with my leather strops.
Balsa, leather, kangaroo, hardwoods and even brass are all just media with varying levels of softness used to hold the diamond particles. I see the paste as just being a convenient media for carrying the abrasive grit so that it may be more easily spread onto the substrate you’ve chosen. Once embedded in the substrate, the particles don’t need the gooey paste anymore.
Whether to use a wetting agent is a personal choice. Does it keep your substrate material pliable? Do you think you need a lubricant? I think it’s well known that while lubricants may make for a smoother finish, they inhibit the abrasive process, making it take a little longer to achieve the same results.
As I said, I haven’t tried balsa, and I don’t use leather anymore either. I have tried paste on hardwood (maple) and on brass and never with a wetting agent. I thought they worked quite well. I prefer the film on a hard substrate. I always felt that film has more of a cutting effect, while paste/spray has more of a burnishing effect.
I usually wet my leather strops with a light misting of rubbing alcohol. I like how it makes the leather grab the steel as I’m stropping. It seems to work best with strops that are well used, with lots of tiny metal filings embedded in the leather so that the surface looks black. With balsa, I also wet the strops but for a different reason - the paste seems to cake on the surface and flakes off when it’s dry. I think the alcohol breaks down the emulsion of the paste and it’s more readily absorbed into the balsa and less comes off on the knife.
I like to feel quite a bit of resistance when I’m stropping. My hypothesis is that once there are adequate metal filings embedded in the leather, along with the micro-abrasive, you start getting the best of abrasion and burnishing together. I believe that the ‘stiction’ helps improve the burnishing action, which really helps with polishing. The alcohol makes my strops grip the blade much more aggressively, creating more ‘stiction’.
All that said, I’m busy experimenting today with stropping (and with lapping films) and starting to unlearn some of what I thought I knew…
Don’t quite get this… If you’re not removing the grind marks (scratches) completely then you’re not done with that stone.
Maybe a picture would better describe what you mean?
Should’ve said “the scratches from the previous stone are all gone.”.
But if you mark an edge with a Sharpie, then swipe it with a stone, you’re right, it will leave “high and low points” but there won’t be any Sharpie left… it doesn’t leave a low point with Sharpie inside.
And if you have an edge that is coarse and mark it with a Sharpie, then sharpen it with a fine stone, and don’t remove the coarse grind, then the edge isn’t sharpened to the level of the fine stone. So if you have coarse grind (and Sharpie) left,then you’re not done at that level. (What I meant before).
But mainly the Sharpie is primarily used just to see where the stone is hitting or not hitting… for example a microbevel that isn’t getting sharpened out or the stone’s path on an edge. Not sure how Sharpie in the low points plays a part, other than what I described.
How exactly are you “feeling” it? Running your fingers along the edge (like Murray Carter’s 3-finger test), dragging a thumb across the edge… etc., and what result do you look for?
To answer your question, yes I feel an edge to check progress… testing it like the 3 finger test. Here’s it described, if you haven’t seen it…
Im running out of arm hair ATM so my fallback has been push cutting phonebook paper because its thin. I watched a video last night where a guy did a sharpening test that was almost like a magic trick 
https://youtu.be/33DM8Q63vM0?t=1288
I don’t think what you’re doing is wrong… from your description it sounds like the way to primarily check for a burr. I’m sure if you use it regularly, you can tell other things too. Nothing wrong with that.
Not really a “feel” method, but another way to test sharpness (and keep fingers safe) is with an “edge tester”…
Bob from Oldawan Tools sells it. Once you get the hang of one, you can tell pretty accurately how sharp your knife is, if there’s any burrs, etc. Here’s the link:
http://www.oldawan.com/sharpen/edge-tester/
something to consider. (You can do some of the tests with a plastic pen… this just provides a little more accuracy and consistency).
I sometimes test my knives like that but than with cigarette paper.
I thought we were talking about feeling an edge and testing for sharpness. Can’t tell that with a scope (just a couple of posts up)…
So…???
Im gonna start a new Topic regarding testing for sharpness…if nothing else it will show some neat new tricks to try! 
The best test I’ve found is inspection with the USB 'scope. I look directly down on the edge and focus on the apex, then slowly move from heel to tip. A truly sharp edge will reflect no light. Even the smallest burr will show up as a glint along the edge. If the edge looks good at 50X and I really want the sharpest edge possible, I will go to 180X and reinspect the full length again. Of course, each blade get the paper slicing test before sending it home.
I will also inspect for a centered apex by holding the 'scope dead vertical and watching for differences in the width of the bevels. If it looks wrong, I’ll roll the 'scope from side to side to get a better view.
BTW, I often use the 'scope as a measuring tool by capturing an image and measuring features with the software. For instance, my wife has a very expensive wrist watch and one of the very small pins attaching the band to the watch fell out and disappeared. With the 'scope I measured the hole diameter and found it to be 1.511 mm. Figured that the 11 microns was probably measuring error and ordered some 1.5mm rods on Amazon, saving about $90 in repair costs.