Hi @ndsugi ,
There are two separate issues here for the purpose of print. Issues with the geometry (non-manifold, not watertight, etc), and issues with the “physical” form of your mesh.
I concur with @tobor8man–the mesh in the form you have on display here will be problematic on a fundamental level for an FDM print process for reasons completely aside from errors in the geometry. In particular the open sections of mesh that are made up of long, thin panels of geometry seem impossible to print as a single piece in the way they are now.
To be clear–even if you successfully fuse all this geometry together in the form it is now in ZBrush or otherwise, the mesh will still be problematic for print. The basic form of this mesh, not just the geometry, needs to be consolidated far more than it is now for the purposes of print. OR the mesh needs to be broken up and printed as a series of small individual pieces that will be fitted together after printing. Prepping a piece like this for that sort of process would be a daunting task.
For the purpose of print output you need to work towards closed, watertight solids with enough thickness for your printer’s capabilities. You also have * a lot* of deeply inset undercuts and overhangs that would frankly be a nightmare to create support for. You want to be working towards chunky, single piece solids with a level of surface detail that does not create overly complicated, deep cavity recesses.
Setting all that aside however, if you produce a mesh of sufficient thickness in ZBrush that is a closed, wateright volume, and that mesh reports no errors with Tool> Geometry> Mesh integrity> Check Mesh, chances are good that the geometry itself will print alright even if your slicer reports minor geometry errors. Those programs can be over sensitive.
Watertightness can be checked with the “Analyze Selected Subtool” report in the Transform palette.
That is purely in regard to errors in the geometry (non-manifold, not watertight, etc). Your slicer could be warning you about issues with the form of your mesh–the issues I mentioned above. Those warnings seem valid to me in this case.
Re: Decimation. Decimation is typically the last step in the process before export. Its primary purpose is to reduce polycount to make the file perform better in programs that may be limited in their ability to handle high polycount meshes. Decimated geometry is difficult to work with for many purposes in ZBrush though, which is generally why it is a final, pre export step. Any errors in your mesh geometry should be corrected before Decimating, as those errors may prevent the piece from Decimating correctly.
This piece has too many long thin sections of geometry to be fused with Dynamesh. That feature is likely to produce a very broken mesh if you attempt to use it on this model in its entirely.
Live Boolean can be used to fuse everything together, but the potential for Mesh Integrity errors is quite high. Geometry> Mesh Integrity > Fix Mesh may correct them, but I consider it likely that any attempt to fuse this geometry together in the form it is now will result in problem sections of mesh that are fundamentally unsound. In that case the mesh may continue to break or generate errors until those sections of mesh are identified and either redrawn in a more stable fashion, or cut away entirely.
What really needs to happen here for the best results is that you need to completely re-evaluate the form of your mesh and consider how to create it as more of a unibody form with surface detail that does not intrude too far into the surface of the mesh. Sort of squint at your mesh and try to see it as a vastly simplified, solid form. How would you carve that solid, one piece form out of a block of wood? For best results, you’ll want as much detail as possible to be simple surface detail on that basic form, and not a series of tiny individual mesh pieces. The cleaner the form of your mesh, the better it will print, the easier it will be to prep for print, and the easier it will be to avoid geometry errors.
In order to do this you may need to retopologize completely new, vastly simplified geometry over the top of your model as a solid piece, and recreate the detail on the surface of that new mesh piece.
In other words…