How to identify the type of coating currently on a surface

When selecting the coating type to apply to a previously painted surface it is important to know whether incompatibility could result. The best way to avoid this is to first identify the coating type currently on the surface. Quick tests can narrow the options down to a level where a potentially expensive compatibility problem is avoided.
Without becoming too technical, dry coatings exhibit solubility, or a lack thereof in certain solvents, and this can be used to identify the coating type on the surface.
The method used is to rub the surface of the coating with a rag wet by solvents 15 times back and forth (known as 15 double rubs), and inspecting the rag for dissolved paint.
If 15 double rubs using Lacquer Thinners as the solvent caused no dissolution of the paint coating, then the product on the surface would be a twin pack epoxy or polyurethane.
Similarly, if a high aromatic solvent dissolves the coating, a water-based coating is indicated.
If the high aromatic solvent does not dissolve the coating, but Lacquer Thinners does, then a single pack solvent based product is likely to be on the surface.
Although this is not a fool proof method of testing a paint film, it does give a good idea of what one is faced with, and helps when selecting a new top-coat option. Costly mistakes are avoided when the correct information is at hand when specifying coating systems to be applied to previously painted surfaces.