It's recommended to watch your watch strap every 1-2 weeks, particularly if you use it often. While it might not seem dirty on the face of it, there will be dead skin and sweat bedded into the strap that you can't see with the naked eye.
Things to Consider When Washing
The main thing to avoid is heat, as Nylon can melt in very hot temperatures.
You also don't want to use any extereme detergents, and this combined with not using hot water will keep the colours fresh.
The Best Washing Option
The best (and quickest) way to wash your NATO strap, is to remove it from the watch face, and use hand soap (or other gentle cleaning products) in a sink of cold water.
You can work the soap into the strap with a toothbrush for speed, and give it a good rinse.
This can be left to air dry flat overnight.
The best alternative is to store the strap in a sock or other tight pocket in clothing, then put it in the washing machine on a COLD cycle.
Then let it air dry overnight again - DO NOT use a tumble dryer, as this could melt the Nylon.
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