Ok i am having my first try at retexturing some models in Portal
I have tried 100's of different types of texture as in, texureing all over, different colour/shade... and so on.

But i want to make it so i don't have to quite out of hammer so i can test the skin on the models.

Anyone here know how to add the texture to a model of "skin types".
You know when you place a model it has the option of "Skin".
I would have to edit the model for this wouldn't I?

Basically, yes you will have decompile the model, add in the new lines to accept the new skins, and recompile the model. All in all it is fairly simple. Here is the tutorial on Interlopers that I found that explains it:

http://interlopers.net/index.php?page=tutorialsid=7557

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Trouble is it doesn't say how to add in a new skin, only how to edit a skin