Modify 2 objects so they can be printed in stainless steel

There is a head of a girl, and separate her hair. they have to be printed separately because of different finish. The face is accurate and with enough meshes the wall thickness must be 3 mm to save money you can remove the none visable parts of the top and back of the head the hair needs to be converted to more meshes eg. finer details the wall thickness of printing must be 3 mm and for the hair tips 2 mm the tail must be hollow The face and the hair must fit together. so in the end I have 2 objects: 1- the hair with a thickness of 2-3 mm 2- a face of the girl without the top/back of the head with a thickness of 3 mm this face fits more or less exactly inside the hair-object