Delete small structures on a surface

Question

I try to modify an object in Blender which was extracted from a CAD file. It looks like half of a sphere with a solid rim and some distortions (something like you can see in the picture)

My problem is: I want to have a flat inside of this object. Is there a possibilty to achieve that without selecting and deleting all the vertices I don't want? It is hard to use border or circle select because there are always some vertices behind the structures I want to delete.
I hope I described it properly.
Thanks!

are you talking about the several peaks we can see? It's easy to delete them, for example with a tool like Grid Fill, you can share your file so that someone can show how to do — moonboots, Commented Sep 7, 2018 at 15:30
Seems like an annoying problem :/ It would be good if the CAD program had choices for different kind of export settings that would make better mesh. — Manu Järvinen, Commented Sep 7, 2018 at 15:47

Manu Järvinen · Accepted Answer · 2018-09-07 16:34:43Z

You can't do much in Blender to have a better mesh imported. Hopefully your CAD program's export settings have more options for better exported mesh.

For that particular object and problem, I'd go like this:

Select a clean faceloop in order to see how many faces there are in one ring (in my case, 128):
Select the absolute center vertex of the object and place the 3D cursor there by hitting Shift+S and selecting Cursor to Selected
Select another vertical faceloop that doesn't have any of the crappy geometry. Make sure to zoom in and add faces to the selection all the way to the cursor, both from top and bottom of the object. (In my case I needed to select 2 triangles near the 3D cursor and from the bottom.)
Hit Ctrl+I to invert the selection. Delete all the faces.
Use calculator to find out what is 360 degrees divided by the number from phase 1. In my case 360/128 = 2.8125 degrees
Press . to use the 3D cursor as pivot point. Select all with A or AA. Do the following without clicking with mouse, moving is okay: Shift+D to duplicate the faces, R to rotate them, Z to use only Z-axis, type 2.8125 (or the number specific to your case), hit Enter. Press , to use the default Bounding Box Center as the pivot point again.
Press and hold Shift+R to repeat last action until you reach the whole circle.
Select all with AA, press W, select Remove Doubles, Ctrl+F and select Shade Smooth to be sure the geometry is nice and clean. In some cases you need to press T or F6 right after Remove Doubles in order to adjust the Merge Distance to make sure all the vertices are welded together.
There you have it, a clean bowl :)