It looks like you may have faces on the horizon of symmetry. Where the mirroring happens, there should be no faces filling the border edges because they will be duplicated on top of each other when using the mirror modifier:

Why does my model look like this after subsurface divide

posted to: Mailbox Modeling II

December 13, 2017 3:28am

Please help.

Kent Trammell replied December 13, 2017 4:43pm

It looks like you may have faces on the horizon of symmetry. Where the mirroring happens, there should be no faces filling the border edges because they will be duplicated on top of each other when using the mirror modifier: