We no longer make Hydrostop. Hydrostop was an effective method of pre-treating dry flies so that they’d float higher and longer. Unfortunately, Hydrostop required a full 24 hours to dry in order to perform correctly. In our experience, this caused a significant reduction in its usefulness to tyers and anglers who would either a) forget to apply it in advance or b) would prefer to apply it streamside. When we developed Fly Dip, we know we had a substitution for Hydrostop. It exceeded the performance of the Hydrostop in its ability to waterproof flies, and it dries almost instantly.
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https://loonoutdoors.com/collections/floatants/products/fly-dip