A very helpful plugin which can make your materials dirty in one click! - You can control the level of dirtiness. - You can make it dry or wet. - Dirtizer supports Multi-materials (you can set random dirt to all materials at once) - You can add your own textures of dirt. (Organize your textures in 'Dirtizer' folder as you want)
Most render systems are supported now! Tested with V-Ray, Mental Ray, Scanline, Corona, Redshift.