After UV paint is applied to the vehicle, the film will remain wet until it is exposed to UV energy. The polymer chains that make up the coating remain short and unconnected until UV light is directed onto the paint film. When UV light is focused onto the paint surface, chemicals known as photoinitiators are activated, which start the cross-linking process in the paint film. In just a few minutes the polymer chains are linked together to form a very large network. This causes the paint to solidify and cure.
This process is somewhat similar to the process associated with exposing photographic film, or more graphically, the exposure and development of Polaroid® film, which was popular several years ago. Nothing happens to film until the film is exposed to light. When that happens, chemical reactions occur that change the film. In the case of Polaroid® film, the whole set of reactions, including exposure and developing, happened in a minute or two before your eyes, just like the UV curing action of UV paints.