Traditional one-layer adhesives are permeable, which may cause sensitivity, faster degradation, and incompatibility with slow-curing resin materials. The two-layer adhesive system, utilizing a hydrophobic second layer to eliminate these issues, is considered as a “gold standard”. Today some one-layer universal adhesives are designed to be as hydrophobic as a 2-layer system. Only these hydrophobic 1-layer systems are as good as the 2-layer gold standard.