if you work with mirroring it can happen that one side of the model has the normals inverted.

blender can draw the normals. did you try this? if they all point outside then the model should export correctly.

blender draws geometry double sided so drawing the normals is the best way to tell if something is wrong.



you can be precise with weight painting too. try to limit yourself to one bone per vertex at the problematic areas. it's always possible to fix such problems with some effort.