Bonding resin to glass usually requires the aid of a silane coupling agent. These products not only initiate the chemical attraction between resin cements, for example, and a porcelain restoration, but actually help bond the glass filler particles in the resin cement to the resin matrix. They also make porcelain repairs possible. Our tests suggest that they usually increase bond strength of one composite to another, but this does not seem to apply to all composites. In the sequence of use, porcelain is either etched with hydrofluoric acid, sandblasted, or both. If hydrofluoric acid is not used, regular phosphoric acid etchant is applied for five seconds (doesn't etch the porcelain, only changes surface chemistry), the etchant is washed off, and the porcelain is dried. Then the silane is applied.
Our tests found bond strengths were not significantly different if the silane was applied for 5 or 60 seconds before drying. The results of these tests appear in the commentary for each product.
WARNING We have also found that silane can interfere with bonding to dentin. Therefore, when applying silane, use a fine brush and limit its application to the restorative material. Do not allow it to run onto the adjacent tooth structure.