How can I get the skin model?

I want to get the skin of a 3D model. Some vertices are hidden because it is inside of the model. Now I just want to obtain the coordinates of the “skin” of the model, How can I get it!!
I began learning OpenGL and 3D graphics recently, may be this question is not clear. thank you for reading the message and give me some advices!!

Why would a model have verteces inside the model?? If it isnt visible, there is no real point of it being there, just taking up memory etc… Maybe you havent explained yourself properly…

Maybe you mean you want the convex hull that encloses the model?

Yes, it is the Convex Hull ! Would you like to tell me how to get it?

Sorry, I have no idea how to calculate one. However, I have no doubt that a search engine will give you more answers than you will know what to do with.

Is this for collision detection?? if so, better to use bouding spheres or other typed of bounging volumes, faster and simpler…