Medium Density Fiberboard, or MDF, is the most common and popular building materials for interior doors in the country. MDF is made of most recycled wood fiber material that is mixed with resin and/or wax. It is the best choice when it comes to interior doors that will be painted because it is resistant to warping, splitting, or cracking. The lack of raised grain on MDF products also factors into its desirability when it comes to painted applications and it is usually considered to be more economical that wood alternatives.