# How to retain watertight mesh after merging two stl using surfaceAdd?

**URL:** https://discourse.vtk.org/t/how-to-retain-watertight-mesh-after-merging-two-stl-using-surfaceadd/5402
**Category:** Support
**Created:** [March 18, 2021, 8:16am UTC](https://discourse.vtk.org/t/how-to-retain-watertight-mesh-after-merging-two-stl-using-surfaceadd/5402 "2021-03-18T08:16:15Z")
**Posts on this page:** 1
**Page:** 1

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### Author: ![rasunag27](https://discourse.vtk.org/user_avatar/discourse.vtk.org/rasunag27/32/3160_2.png) [@rasunag27](https://discourse.vtk.org/u/rasunag27)
#### Post date: [March 18, 2021, 8:16am UTC](https://discourse.vtk.org/t/how-to-retain-watertight-mesh-after-merging-two-stl-using-surfaceadd/5402/1 "2021-03-18T08:16:15Z")

</div>

Dear all,

I have an stl mesh which is watertight mesh (Checked with surfaceCheck utility of OpenFoam).

1. I splitted this mesh to two stl’s using Paraview filters. (STL \> GenerateNormals \> Connectivity \> Threshold \> ExtractSurface).
2. I saved these two stl files. Then I merged these two stls using surfaceAdd utility. (I merged again because I need separate patch in snappHexMesh).
3. But, after merging, surfaceCheck shows the mesh is not closed. How do I retain this?

For reference, I have attached the zip file of stl.

[stl\_mesh\_files](https://drive.google.com/file/d/1LtkRsAYxXOJNwxmSaoZcrs-o6SWb1DUb/view?usp=sharing)

Any leads will be appreciated.

Regards,

Sunag R A.