Help! Cant reach some areas in weight painting

Ok so im not to good at modeling so i guess when i made my human model the mirror overlapped and created little bits and pieces inside the body, so i am now weight painting it and it seems to be working fine until i test it and pose the model. So and when i try to pose him the parts i painted work fine but those little pieces inside the mesh didnt get painted so they stay behind creating an amazingly ugly thing!! Here are some pictures of it, if someone can help me i would appriciate it soooooo much!

The image is sideways, just right above the red is the spot im talking about

Use a mask modifier to make certain parts invisible.

Create a vertex group of the parts you want to hide (you can do this by selection, so you don’t have to paint). Add a mask modifier and assign that vertex group. During weight painting you can easily invert the mask and enable/disable it using the modifier functions