Fill space between inner mesh my problem is the following:
I've made an object in blender and now I've begun applying material but I've noticed that I've added multiple meshes between the innermost and the outermost. This is quite annoying for the part which should have a glass material (the "vizor" as it is visible in the captures). In fact I've first tried to select face without limit selection to visible but as there is the back of the object which is also selected this have not really worked.

EDIT: Ok I'll try to clarify my issue, I've uploaded another capture, this time only one  half of the mesh. As you can see there is multiple inner faces especially in the red rectangle (in the image attached) and those inner faces are those I want to assign material on (or delete all other than to outermost)
I went with the delete all but the outermost option (well, I kept two layers of the four you had). You have a circular opening in the top of the inside of the helmet. Select all the faces of its rim. I used circle (C) select for this and panned around until all were selected.
  

Delete these faces (X then F)
  

Select one face on the unwanted geometry
  

Select linked (CtrlL)
  

Delete everything that's selected (X then F)
  

It now looks like this (with all selected, zoomed out and limit selection to visible disabled)
  

I hope this helps.