What exactly is Polymer?
Polymer is short for polymer emulsion, which indicates the chemistry involved in making the resins used to create paints and coatings. This is the binder or glue responsible for forming a continuous film, holding the pigments and other materials in place. A polymer is a large molecule made up of monomers, and the ones we use are called "butylacrylate" and "methylmethacrylate". In short, these polymers are floating around in water and we add thickeners and other additives to make the kind of medium, gel, paint, gesso, or whatever else we want to create. This is essentially what you have when you buy the paints or the Polymer Medium.

Once you brush some out, the water and additives start to evaporate out, and the polymers floating around start getting closer and closer. Some additives are like traffic cops and make sure everything lines up perfectly. When the film becomes dry to the touch, everything is in place. However, there is a slower curing in which the remaining water and additives leave the film and the polymers get so close they go from their round shape to a hexagonal one. Once the film reaches this, you have a strong film, and the best clarity. You may have seen this happening as the milkiness goes away and a couple of days or weeks later, the gloss and brilliance really come out.

What is long and short paint?
Short refers to paint having a smooth, buttery texture. Resinous or stringy are opposites but are not defined as long. Long can sometimes refer to a high oil content, more flow or fluidity to a paint, but the term “long” usually is a characteristic term given to a higher proportion of oil in a resin/oil medium or in some alkyd resins.