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try 600 if it does not work well, go up and reapply the paste.

You can paste it but still not going to sharpen the razor, just want to make sure you understand that strops do not sharpen, they can somewhat refresh an edge that is a bit off.
The other side from...

In order to sharpen you have to remove enough metal, that happens on the 1-4k stones after that is smoothing the edge pretty much.
Leather does not abrade enough to sharpen, to test that just grab a...

Strops do not have sharpen side as far as I know. The top side is the stropping side and that is it.

Depending on the condition you can start from 220 or 400 and go all the way up to 2000 grit.
Another way of redoing the surface is to use Pumice stone, then 600-2000 sand paper.
The job is messy,...