403 Microfibers is West System's cotton-fiber bonding filler: stir it into mixed 105 epoxy to build a non-sag structural adhesive for fillets, gap filling, and glue joints where you need strength more than a pretty sanding job. Add it to mixed resin until you reach mayonnaise-to-peanut-butter consistency depending on gap and vertical hold. The cured mix is off-white, tough, and deliberately not a fairing compound; keep it in the joint, not on the surface you plan to sand. Works in polyester and vinyl ester too, though West markets it for the 105 system. Shop-quantity alternative: our 1/32 in milled fiber does the same structural thickening with chopped glass instead of cotton, in any resin, at bulk jar pricing, and fumed silica handles the anti-sag half of the job. The spec table below compares all three.
