I think using Cloth Surface for thickness like this video (14:00) has issues. I'm around chapter 15 and start giving objects PBR materials using OctaneRender, the refraction of the fishbowl is wrong no matter how I tweaked the renderer, then I found by using positive values in Cloth Surface's Thickness parameter solves the issue. In the end, I recreated the fishbowl with splines with what I've learned from this series, and I found it more intuitive and precise, and it works pretty well when render. I absolutely love how this series cover as many tools as possible, but I just think choosing this tool for this purpose may not be optimal.

@class-sympathy said in [Help] Introduction to Organic Modeling Techniques:

Solved: For anyone struggling, make sure isoline editing under options is switched on (alt+A)

This solved my exact problem. For some reason this option is disabled by default in 2023.2.0 while all the video tutorials I watched have this already on.