Abstract Surface defects cause the measured tensile strength of glass and other brittle materials to be significantly lower than their theoretical values. Coatings can be used to “heal” surface flaws and modify surface properties. Here, we describe an online process by which a nanometer-scale hybrid coating layer based on styrene–butadiene copolymer with multi-walled carbon nanotubes (MWCNTs) and/or nanoclays, as a mechanical enhancement and environmental barrier layer, is applied to alkali-resistant glass (ARG) and E-glass fibers. Our data indicate that the nanostructured and functionalized traditional glass fibers show significant improvements in both mechanical properties and environmental corrosion resistance. With low fractions of nanotubes (0.2 wt% in sizing), the strength of healed glass fiber increases by up to 70%. No apparent strength variation appears for nanoclay-coated fiber subjected to alkaline attack. The adsorption isotherms of moisture vapor on the fiber surface have been analyzed and an assessment of changes in the nanomechanical properties of the fiber surface is provided. It is shown that the sorption of moisture is reduced by the presence of nanoclay particles in the coating. We introduce a healing efficiency factor and conclude that the coating modulus, thickness and roughness are responsible for the mechanical improvement of fibers.