I would think of a decal on a fly rod as not much different than an adhesive label on any other product. Not much reason to preserve it for function, nor for value on a run-of-the-mill, everyday use product. But there is no reason not to protect it, either, especially if you're leaving grandma's jam in the refrigerator for a lot of years and might forget what it is. The best way, at the expense of 100 percent authentic, is too clean it thoroughly, spiral wrap with pale, fine thread, and finish with spar varnish or a thin-two part rod finish. Spar will work well on the brown Fenwick blank, but you might prefer one of the crystal clear coatings instead.
Here's the idea on a print label (also happens to be a Fenwick) that I wanted to protect, since the lettering wore off easily. No need, no reason, but I did it anyway.
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