Thus, sizings and finishes have been developed empirically from a handling viewpoint rather than in terms of interfacial energy. Another problem is that manufacturers are under pressure to reduce inventory. They don't want to offer an extensive line of sizings, but rather they want a generic sizing applicable to many situations so they can offer fewer kinds of products. Thus, epoxy compatible size from two manufacturers may have completely different chemical compositions. Furthermore, neither is guaranteed to be truly compatible with the particular epoxy resin being used.
Manufactures keep an edge by treating sizings as proprietary and keeping their compositions secret.
A further complication is the great complexity of commercial sizing systems. Here are three compositions for glass fiber size which appear in patent literature. All three sizings are water based. All three have a coupling agent which provides corrosion protection for the glass surface. Each has a reactive group to interact with the matrix.