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Did you use a belt-sander?

The problem with sand paper on a strop is the risk of imbedding loose grit in the leather. One grain can ruin an edge.

I prefer to scrape leather with a sharp cabinet...

Those loom strops came pasted with Chrome Oxide, (Green) or Ferrous Oxide (Red). Two-sided pasted strop are easily contaminated.

You can remove or smooth out paste with lighter fluid, WD40 or most...