I sometimes work with .5" MDF panels I join together in 90 degree, glued butt joints. These get sanded, treated with sanding sealer, spray primed, and spray painted, producing a joint that, at least initially, is invisible. Occasionally, however, some of these joints reappear within a day or two as one side rises (or the other sinks) slightly, less than .005" but noticeable. What causes this? Is there any way to prevent it? Thanks.