can see inside object that is supposed to be fully opaque

Hello, i am trying to implement a fade on a brain surface, but when i render the fully opaque brain, using a transparent shader, i get weird effects where i can see inside the brain (ie, i see parts of the brain that should be occluded). when i render with a non-transparent shader, i get the desired effect at full opacity, but obviously transparency doesn't work.

below LEFT: brain at full opacity with transparent shader, below RIGHT, brain at full opacity with non-transparent shader.

does anyone know how to implement a fade while keeping it as it is on the right, at full opacity?