The effects of sun, wind, rain, frost and the weather in general on an old and unwanted domestic door. Perversely it also looks as if there was never any undercoat used in the original painting process too. The boss and I have seen plenty of paintings and artworks trying to achieve this finish but we have never seen them manage this level of success, a crackle 'finish' that has probably taken months of exposure to the elements.
Perhaps the answer is don't be precious, simply put the artwork outside and let the elements go to work. You simply can't rush Mother Nature and the natural sequence of events in a natural process.