I used to work in a joinery shop where we handled a lot of MDF, most of which had routed mouldings. We always used a fine, oil-based wood filler, ususally applied with a finger. This was then rubbed down with 200 grade (or finer) sand paper. Sometimes the process was repeated more than once to get the routed edges as solid as the face. It's also worth giving the faces a very light rubbing with very fine paper just to allow the primer to key in and remove any grease spots from handling , before priming. Indeed, it's worth washing your hands before handling the stuff. Also, I always use a specialist MDF primer, rubbing down Uspecially those edges) between coats. As with most things, the quality of the finished job is only as good as the effort you put into preparation
On a safety note, MDF dust should always be wiped off with a slightly damp cloth rather than blown off, it's pretty evil stuff.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!