Hi Henry! I’m glad mesh guru Martin found the solution for you 🙂
Yes, in many situations double vertices are the villains. In bad shadings it can be flipped normals. A third thing (maybe not that common) is to check for manifold mesh. You can do that in edge select mode and in the select menu select all by trait, and non manifold. If then your selection is not empty, it means you have a non manifold mesh. And that in most cases we don’t want. So you then need to fix each parts of the selection until you get an empty selection when you click non manifold. I hope that made sense. Good luck!
