With some elbow grease and time, i started to sand the model in a way that you cant see the marks left by the "slising procces" (from CURA in exemple) . i used many sand paper size to achieve such "shiny" case 120 => 240 => 320 => 400 => 800 => 1500 => 3000 . I think using sand paper only, was a mistake : for the next case i printed yesterday, i'll use a proper grater because PLA from E-iCE is warming up very quickly even without applying so much force on the sandpaper .
i took me 5h to sand the whole case . but i added some more details using a metal grater . like on thoses pictures on the attachement .