I work on 6mm MDF which has a minimum of 4 coats of white gesso on before I start painting it. The back of the MDF is sealed with a PVA and water solution. The painting is done in layers. This requires a lot of patience, because a new layer is not started until the previous layer is dry. To help speed up the process I use the medium Liquin (which also adds a gloss to the artwork)... patience is the key with my style of painting, especially as I like to add plenty of detail, most of my artworks are done over a period of time of about two weeks, although some can take months.