Combining multiple RegionPlot3D regions

Questions
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1

For 3D rendering purposes and minimizing .stl file size, I need to generate an object with different values of PlotPoints and MaxRecursion for different parts of its geometry. For example, I want to define a shelled sphere with a groove cut out of it. The inside and outside surfaces of the sphere can be low resolution (Plotpoints → low , MaxRecursion → low ), but the equation-driven groove needs to be high resolution. This cannot be achieved in a single RegionPlot3D[…] command, so I tried using “Show”, and excluding the groove region from the shell region using “not”, but it doesn’t work:

sphereshell = RegionPlot3D[.....,PlotPoints->10,MaxRecursion->1] 
groove = RegionPlot3D[.....,PlotPoints->1000,MaxRecursion->15] 
Show[sphereshell,\[Not]groove]

Any ideas?

2

Sometimes it helps to reverse the order of entries in Show.