The following object, in obj format, is 1.6 mm thick and I want it to be 0.8 mm thick. How do I decrease its thickness (from the inside, so that the outward appearance is still the same)? When I go to Add Modifier->Solidify, it only allows me to increase the thickness, but not decrease it.
Decrease thickness of object
I have made a half sphere and added some solidify modifier and applied to it. To decrease thickness or shrink down the thickness. In edit mode -> select all and press Alt+S or Mesh -> Transform -> Shrink Fatten
As soon as u click on or better press Alt+S like we scale we get a arrow to increase or decrease the thickness. After click u can press F6 for operator panel or adjustments. Click Ctrl+N to flip normal.
I guess you could simply select (eg) the internal shell, place the 3d cursor at center, select the pivot method for transforms to "3d cursor" and scale the selection:
here I scaled towards the sphere center
and here the opposite
You can't "snap" the scaling to the "middle" of the current thickness, but you can try to determine the needed scaling somehow: eg, try to scale it by increments (or using vertex snapping tools) until the inner shell matches exactly the outer one, note the scaling factor, and then undo, and scale again using a desired percentage (eg 50% of that to go from 1.6 to 0.8).
here I snapped the scaling to a vertex of the outer shell,
note the scaling in the bottom header: half of that amount, entered by keyboard while scaling, will do the job.