shaving ther plastics behind the door

ive been workin on one of th eplastics i have a mazda and im practiing doin body work ive never tried it so im tryingon my parts truck i use the kity hair and then the bondo and everytime i sand the bondo and primer it you can still see the lines where the vents were. its smooth i dont know if i have to add soem more bondo what do yall think?