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Zmodeler extrude all polygons doesn't perform, how do I thicken inside of this?

Hello everyone. I would grealy appreciate some help with this please. The Create Shell of using another subtool to start the shell is fine for 3d printing but for the final process of making jewelry (which I do as a hobby), models need to be much more open in the back for proper casting/investing.

Anyways, I’m new to Zbrush and I saw a video where someone used the Zmodeler “extrude, all polygons” method to thicken the inside of the walls. But I cannot get the function to perform. My model is currently in Dynamesh mode, is that Ok for Zmodeler? Also, do I need a separate polygroup for the inside of this hummingbird? I don’t know how to make a polygroup only for the inside of this.

Or is there an easier way to thicken the walls of this hummingbird?

Thank you for any advice. I’m attaching the file if anyone wants to check it out and see if there is an easy way.
I love Zbrush so far even though it’s extremely challenging for me to learn it.

https://www.dropbox.com/s/na9hidflz0h6us2/humming%20bird%201mm%20grid%20project%20ready%20for%20inflation%20extrude.ZPR?dl=0hollow needing extrude.jpg

I read that for high poly mesh, it’s easier to use extract. I’ll try that.

The simplest way is probably to use the DynaMesh Create Shell option. Joseph explains the method well in this video, starting at about 46 min: