Well, in making hand coatings, I usually stress how the emulsion is made and coated and don't delve too deeply into keeping. Although the emulsions keep for up to 1 year with no appreciable change and the coatings keep for up to a year with no appreciable change, there are things you should know about "real" products which will influence your work.
All real products have an overcoat. This overcoat is needed in machine coatings due to the fact that the winding operation at the end of the machine causes cinching of the film or paper and abrasion. Both of these can cause scratch marks and fog. Also, there are some things best not placed in the emulsion layer during coating, but rather are placed in the overcoat.
So, here goes.
An overcoat is used to insulate the emulsion layer from cinching caused fog and scratches. It is NOT needed for hand coatings because there is no rough abrasion of front to back surfaces during the winding process, but on a machine you are required to have either 2 coating stages or a C2 multi hopper coating stage for B&W or color products.
In addition, lubricants are added to the top layer to help eliminate any cinching that may take place.
But, in addition, this top coat may contain hardener, antioxidants, buffers and mattting agents. The hardener was added to the overcoat because in many cases, hardeners such as formalin were reducing agents which could fog the emulsion. Adding formalin to the overcoat reduced hold induced fog.
Antioxidants and buffers were added as preservatives and could not be added to the emulsions in some cases due to the possible effects of pH shock on a hot emulsion or the fact that antioxidants were not good during the hold.
Matting agents were used to create just the desired surface and so had to be on top.
Well, there you have a minor introduction to overcoats and things not really brought to the fore before in this forum.