Hard Mason is the material of choice for monofilament weedguards on flies. It is a stiff material and, unlike some other brands/types of mono, changes very little when wet. As a starting point, for saltwater applications with hook sizes 4 we typically use 16lb mason and go up to 20lb on #2 to 1/0 hooks. Mason is also the favored IGFA class tippet of many record-seeking tarpon anglers.