Why has a gap or paint cracking appeared at the joins?

As with solid wood, MDF takes up and retains moisture, thus its moisture content depends on the relative humidity and air temperature in the surrounding environment. As the moisture content of MDF changes, it is subject to dimensional changes. There are many variables which can ultimately affect the performance of timber, including the surrounding environment, quality of installation and quality of framing. But managing changes in the environment humidity levels and moisture content plays a very important role in achieving the best results from the product. In environments that can experience large changes in air temperature and humidity you may experience movement in the panel, either expansion or shrinkage.