[Gmsh] mesh refinement

[pds58 at student.canterbury.ac.nz]

Hello,

i have little experience with gmsh, 
however i wish to make alterations to an existing model that has a defined characteristic length over the current model
which currently is simply a merged stl file, defining the surface loop, and then meshed from boundry.

Currently i have a 3d surface defined by an stl file, however i wish to include a finer mesh in a 2mm deep layer from the surface
rather than it just being homogenoeus. 
my current idea is to merge two different stl files into the code, defining the outer 2mm layer, and another defining the inner layers.
is this a viable use of the stl files and merge function?
code roughly as follows...

merge "outervolume.stl";
Surface Loop (1)={1};
Volume(1)={1};

Mesh.CharacteristicLengthExtendFromBoundary=0;
Mesh.CharacteristicLengthMax=0.0005;

merge "innervolume.stl"
Surface Loop(2)={2}
Volume(2)={2}

Mesh.CharacteristicLengthExtendFromBoundary=0;
Mesh.CharacteristicLengthMax=0.003;

note that the overlap surface of the stl files would be comprised of the same vertices. 

or is there some post processing, that enables the mesh to be refined in certain areas?

please forgive this crude code, 

thank you 

Paul Simpson