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When adding thickness all the details on the outside are added to the inner side too. Using edge dissolve where it is needed to remove the extra geometry in order to allow the inner side of the object to be more realistic.
Once the holding edges are inserted, select the inner bottle geometry and copy it into its own mesh layer, and cap the top opening using a Close Polygon Hole. The liquid needs to be scaled down ever so slightly so it is not directly on top of the bottle geometry.