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Opinels do well in dirt and sand as their pivot and lock ring aren't affected by grit as much as lock backs and liner locks are. But Opinel's become more challenged by being wet constantly, as the wood can swell making the pivot tighter.
IMO, the biggest thing to grasp about an Opinel is that it comes from another country and, really, another time. There is a bunch of common sense lore about how to use and maintain that style of knife that we, as modern consumerist Americans, don't know or understand. There are countless threads on this forum about how to take apart and clean various modern flippers and plenty of discussions of people's favorite lubes for knife actions. None of this applies to an Opinel. Opinels require a different set of approaches.
IME, an Opinel can be used out of the box in most situations just fine. If the joint becomes tight, grab the knife by the collar and whack the butt end of the handle on a hard object. A boot heel works fine. In France, that sound is called the "coup du savoyard". Anyway, this will usually pop the blade out enough to get a better grip on it.
IME, the best lubricant for the joint is Johnson's Floor Wax paste available at most hardware stores. Carpenter's wax or bees wax will also work well. Stuff as much wax as you can in and around the joint and melt it in with a heat gun. This will help the Opinel stand up to water betting and will help reduce binding. But it won't make it into a dive knife. If you like to swim with a knife in your pocket or are doing active water sports, there are better choices, IMO.
The #8 is the most popular size and still small enough to work for front pocket carry. I prefer the #9 for general carry but I carry my EDC knives in my rear pocket.
IME, the Sandvik Inox is so good there is no reason other than tradition to get the 1086 Carbone.
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