1. An image forming apparatus comprising: an image forming unit configured to eject droplets from the image forming apparatus, and to form an image on a surface of a printing medium; a pre-processing unit configured to apply a pre-processing liquid to the surface of the printing medium before the image is formed by the image forming unit; and a post-processing unit configured to apply a post-processing liquid different from the pre-processing liquid to the surface of the printing medium after the image is formed by the image forming unit, wherein the pre-processing unit applies an amount of the pre-processing liquid which is determined based on a type of the printing medium and the post-processing unit applies an amount of the post-processing liquid which is determined based on the type of the printing medium, wherein the application amount of the post-processing liquid is reduced when the application amount of the pre-processing liquid is reduced, and wherein the application amount of the post-processing liquid is raised when the application amount of the pre-processing liquid is raised.