I was reading about the oxygen-inhibited layer, and I learned something new.
When light is introduced to any light cure resin (or adhesive), it activates light initiators, creating free radicals that will start the polymerization process. Oxygen in the air will compete for these free radicals, creating the oxygen-inhibited layer on the top 10-micron surface of the resin. Not only will the oxygen compete for these free radicals, but the free radicals prefer the oxygen molecules over the resin monomers. I always thought if this layer is covered by more resin, the layer will disappear as the resin monomers continue their polymer chains. However, I discovered this could only happen if the oxygen-inhibiting layer is thin, less than a few microns thin. Even then, the oxygen will not release the free radicals, and all that is happening is free radicals from the overlaying resin reaches the monomers that were unreacted in the bottom layer, continuing the chain formation through the inhibiting layer.
If your oxygen-inhibiting layer is larger than a few microns, this layer will not polymerize and leave a sticky, uncured layer behind. Now, this may be fine between layers of composite, but it is the reason we do not recommend air-thinning the adhesive layer in a 2-bottle adhesive system. The air will introduce more oxygen to the adhesive surface and increase the oxygen-inhibiting layer. Unfortunately, if there is solvent in the adhesive, air must be used to evaporate this solvent. This is one of the reasons we want to use a 2-bottle adhesive system with a solvent-free 2nd bottle.