27.

Add a solidify modifier and set the thickness to 0.2mm. In the right corner on the bottom you will see the Material Index Offset and two fields with zeros in it. These fields are telling the modifier which material to use. The first field is for the back side of the mesh and the second is for the rim. The number zero stands for the first material on the list of materials, the number 1 would be the second material and for the third you need to set it to two. And now you can see that the inside of the guitar is using a different material than the outside.