Why does my model look messy and contains dents when I turn on the multi-resolution modifier?

Question

I wanted to see what my model looked like so far in sculpt mode with the multi-resolution modifier and it looks like total garbage. I'm new to blender so my model does have a lot of overlapping vertices and edges but I'm not sure that's the reason. During my last attempt, I thought it was because the image was asymmetric so I used mirror modifier to make the model totally symmetric. I know that the model is very messy in it of itself, but I'm just trying to narrow the problem to specifics. Honestly, I have no idea what's happening.

Here's the file I'm struggling with:

Before I start from scratch AGAIN could someone give me a clue as to why this is happening?

Thanks

Answer 1

There are 3 main issues that can cause problems for Catmull-Clark subdivision (the algorithm that both MultiRes and Subsurf use).

1. Non-quad faces. When modeling in general try to use all quads, i.e. faces with 4 sides. Subsurf requires quads or tris (3-sided faces), so any non-quad faces will be automatically triangulated which can give undesirable results. An occasional well-placed tri can be fine, but in general stick to quads.
   
2. Inside geometry. Geometry inside a mesh should be avoided at all costs. Inside geometry occurs when an edge is shared by more than 2 faces. This can cause major issues for Subsurfing. (It seems to be the biggest problem in your case.)
   
3. Stretched faces. Try to avoid extremely non-regular faces, i.e. long, extremely non squarish or equilateral-trianglish faces. These can create some messy stretched looking and pinching in the resulting mesh. Your faces don't have to be perfect regular polygons, but long skinny faces will cause problems.
   

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As an aside, your normals are a bit messed up (indicated by the darker faces). While normals don't play a factor in Subsurfing, messed up normals can cause a lot of other problems. After removing inside geometry you should select your entire mesh and hit Ctrl+N to recalculate the normals.

Answer 2

Save your current scene to avoid destroying it more.

Go in to wireframe mode. (Z to jump in to solid and again for wireframe).

If your mesh is messy and have vertices inside it delete them and try to clean up your model if you have an unnecessarily rough shape. The smoother the shape is the better it will work.

Ignore the world node it is not relevant