For anyone interested in better control of the hair.. render the scull cap and hair on a separate layer from the model, you also need to move your lighting and stage setup to a separate layer..
On your render tab you need to scroll down and activate transparent, so that the hair will have a transparent background

- transp.PNG (28.94 KiB) Viewed 7127 times
Now you set up your scenes on the scene tab, so firstly you need to add a new scene layer, the first one is the model, so check the model layer and also remember your lights and stage.
Now move to the hair layer, here you check the hair layer and again remember the lights stage, but you also need to mask out the model layer.
also you can enable different passes as shown in the example, however i never used them in the node setup ( i would normally used the glossy direct, to control glossiness of hair, which would be added via a colour mix node.)
render your image, then move to the node editor
once in the node editor you can now mix the model scene with the hair scene via an alpha over node - in my example my layer with the lights never had a background so i added another alpha over to make a blue background - if i didn't do this then the image would have been on a transparent BG. if you have a stage setup on the lights scene then this node is note necessary.
And here is the result - blue BG, model is there and hair is there - in this example i didnt even have a texture for the hair thus before the node setup it was infact purple (blenders colour for missing texture file) and there are so many other things you can do from this point with all the other passes, like i said i would use the glossy direct which you mix in with a colour mix node just after your hue saturation.