What is an MDF or HDF door skin, and when is it used?

MDF and HDF door skins are thin, dense fibreboard facings moulded or routed into panel profiles and then bonded to a core frame. They give a perfectly smooth, paintable surface with no grain telegraphing, which is why they are the default for painted flush and moulded doors. They must be sealed on all six sides; unsealed MDF edges are the most common cause of swelling complaints.