Hey Bugs...
This has been brought up many times about lining a sheath with material of some type.
Lets say for example that you line the inside of the sheath with the softest leather in the world...
You cut something with your knife, lay it down for a second then resheath the knife.
For arguments sake,, lets say when you laid the knife down, it picked up a tiny grain of sand,dirt, or piece of crud in general. You've just transfered that grain of sand into your sheath.
The million dollar question now is...
How, do you get that grain of sand out?
You may be able to blow it out,, but you'll never be able to wash it out.
I don't care if you line a sheath with the fluff off of a duckling a$$, it will trap dirt and it will scratch your blade.
Cleaning a sheath out on a regular basis is the only way to reduce the risk of scratching. My prefered method of cleaning is water under high pressure; like that from a garden hose or similar.
Air will more than likely work, but I tell my customers to use water,as everyone will most likely have access to a garden hose of some type.
As far as putting something into the sheath, such as grease, oil, silicone,, I don't recommend it for the exact same reason...
At times under certain circumstances I will spray WD-40 or silicone or Armourall into a sheath to make the inside of the sheath slick,, but I will also wash out as best I can as much residue as possible.
The best thing to do is keep your sheath as clean as possible and wash out frequently..
This would be the extreme case,for a knife that you want to keep as shiny as possible...
If you are Very concerned about scratching then sheathing the knife in Anything may cause you problems.
Synthetics get bad press constantly for this. It's not the plastic,, its the way the sheath was made, the care taken to clean the inside after manufacturing, and care of the user in general.
A year or so ago I wanted to see what I could find inside one of my well used leather sheaths. The knife is an Old Timer similar to a sharp finger. The sheath is leather pouch style with no straps or cover.
I cleaned the sheath out onto a white piece of paper. Inside I found deer fur,assorted seeds, a small stone,many grains of sand or general grit, and a crushed and mangled .22 casing. Who knows how that got in there.
Keep in mind that I have had this knife since I was a teenager, it has never been cleaned and has seen major hours of use.
If you have used a sheath for that long without a cleaning,, you may find similar results.
Keep it clean,and you shouldn't have a problem.
ttyle
Eric...
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On/Scene Tactical
Leading The Way In Quality Synthetic Sheathing