When composite is light cured, the final layer is referred to as the oxygen inhibition layer. This means oxygen interferes with the polymerization of the composite, resulting in a sticky uncured final layer.
There are two main ways to overcome this and remove this final layer.
One method is to finish and polish the composite restoration. By doing this you will essentially remove the outermost oxygen inhibited resin layer. However, by doing it this way the dust from finishing and polishing can be difficult to remove from the restoration. Additionally the fine resin particles collect in the burs and discs used during the finishing and polishing stage, rendering them useless.
Alternatively, another method is to do the final light cure through a layer of glycerin gel.
To do this, place the composite as you normally would, lightcuring incrementally as normal. Once you have placed the final composite layer lightcure the restoration. Then place some glycerin gel over the cured restoration and lightcure again – this is the final cure through glycerine.
Now rinse the glycerin off with water and finish and polish the restoration as normal. The result is a surface that is easier to polish and harder.
Park, Hyun-Hee & Lee, In-Bog. (2011). Effect of glycerin on the surface hardness of composites after curing. Journal of Korean Academy of Conservative Dentistry. 36. 483. 10.5395/JKACD.2011.36.6.483.