Tensile strength of E-glass fibres have been analysed using a bimodal Weibull two parameter cumulative distribution function. The bimodal character were associated to surface and internal flaws, respectively. Influence of silane coating as well as industrial sizing on the flaws distribution was quantitatively characterised. A qualitative interpretation of their effect based on a combination of probabilistic and deterministic approaches were proposed in terms of crack healing. It was demonstrated there that mechanical testing of fibres can be used as an indirect observation technique of the consequences of the surface treatment. An analogy is proposed between the sizing treatment of glass fibres and the strengthening of silica glass by means of hybrid organic-inorganic coatings.