Edge-work with thin leather

[Nedscore]

Hi everyone!  I am trying to burnish/close/round the edges of my calfskin leather patches before machine stitching them on to denim.  I bought the little wooden tool and the tokonole, but the leather is very thin and has been quite flimsy and I have a feeling this is done a different way.  I have attached a sample photo that depicts another brand, which has managed to soften/close the edges of their very thin leather patches.  I have heard the term "edge paint" floating around, but this picture makes it appear like some physical was done to the edges, perhaps in addition to edge paint or some other gum/glue sealant.  Please let me know if you know how I can achieve this finish.  Thanks!