How to cut model without leaving a giant hole in the mesh?

I’m attempting to follow this tutorial but I’ve hit a snag.

Once I seperate the two parts, one side ends up with a hole where the other side had been. Eg. I cut the head off and now the neck/body is one solid part, but the head is now hollow or vice versa.

How on earth can I fix this? My mesh is too detailed to just select the outside and seal it. I can’t select all the nodes around the edge and even when I try and fill it, it messes up. This isn’t a new issue for me. I’ve come across this bug a few times when using the boolean modifier in the past and I’ve had to abandon models I’ve been working on because I haven’t found a fix. I’ve tried opening and closing blender and cutting it multiple times. The same issue happens, one side, either the head or the body becomes hollow.

Now, I had this issue with the last model I needed to cut and eventually gave up and cut it via Nomad Sculpt, where I made the model. It’s not a great option because I had to just eyeball where the cut should be for both sides and delete the side and save it seperatly. The file came out fine in the end, but I want to cut it with precision and add notches to make it easier to glue together. The way I did it wasn’t great.

Using Blender 2.83. I can’t update it until I finish my current project (not this).

Can you post a part of the model? Easiest way to fix it is understanding what’s causing it. ‘Hot Lining’ is the most common cause where a vertex, edge or face is overlapping precisely with the bool object.


If you are using simple straight cuts there are several ways to do this…
Booleans work in most cases…the alignment notch could even be modeled into the cutter.
Using knife project can also work using a plane (or other mesh) as a cutter ( but requires filling by hand which you are already having problems with. There is also Bisect ( under the Knife Tab, hold select for drop-down…it works really well, and will even auto-fill the area…but you would need multiple instances of the main mesh as it removes the outer part…ie: destructive., drawbacks are hard to align as the beginning of the cut must be off the mesh, it also will only do straight cuts…

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Boolean does not work at all, gives me the same result, this is an issue I’ve had with simple models as well, the faces just vanish/glitch and cannot be fixed. Bisect just isn’t working at all. It’s not allowing me to just cut a small part of my model, it cuts through the entire thing no matter where I try and place it, that’s how I started out and ended up on the video above. The knife is just going all over the place, works well on planes and simple objects, but not on this model.

maybe your mesh topology is causing issues.

Ok, another weird thing. I just tried to make a boolean square cut in the middle of my object, instead of making the hole, it just merged the block into the main object. Now I’m very confused. What in the hell is happening?

are your mesh normals correct?

Seem to be. I made my object look like a fuzzy cactus when I turned on Display Normals.

Does your mesh have holes somewhere? This can also cause problems with booleans

So, this was it. I checked the mesh again and even though it looked good, when I checked it via face orientation, I could see the model glitching in weird places. There was no way I could fix it. So, I went to sculpt mode, remesh and that seemed to do the trick. Unfortunetly the remesh is taking out detail so I’m not sure how to proceed from here. Hopefully someone has some advice to preserve my original model when I do that!

If Boolean isn’t working at all or is always messed up…It points to not doing it right for what ever reason…

The mesh I showed is your typical sculpted mesh .STL ( they are always a mess)
here I used a Cube-sphere with boolean difference right out of Bool-tool and it works…a few steps after to do some clean-up around the edges…but a printable result…

Again and like I said, bisect is hard to get right…
but works for simple straight cuts…

The knife Tool is the same and just has a few more steps to cut an object shape through another…sometimes with a bit of cleanup…( always with STL files)…

Unfortunetly the remesh is taking out detail so I’m not sure how to proceed from here. Hopefully someone has some advice to preserve my original model when I do that!

Yeah, a few things you could do/ check:

  • With your original mesh try to remove doubles in edit mode. Glitching sounds like planes lying on top of each other.
  • If that doesn’t help then there are also a few add-ons that can point out bad topology and clean it up.
    1. A built-in one called 3D-print Toolbox.
    2. MACHIN3tools. It has a tool built in called clean up. Press 3 when you’re in edit mode
  • If these addons don’t help for some reason or you don’t want to use add-ons you can use your remeshed object and add a shrinkwrap modifier. You need to have both your original mesh and the remesh. Project your remesh with the modifier to your original like so
    You can even add a sub div modifier before the shrinkwrap to preserve more details. Depending on how many polygons your remesh has.