Hello,
Iam modeling a car body, which has a geometry that is too problematic to just add thickness. It makes a lot of intersection and bad geometry The goal is to have a shell for 3D print for toy car. Is there a way to achieve this in blender?
It depends on your specific geometry. Screenshots would be helpful 
It kind of does not matter how the geometry looks, the problem is always the same, intersecting geometry.
Perhaps you can just add thickness to those parts that are more visible and leave the rest as is…
For instance the fender, you could thicken the edge and leave the rest as is…none of the front area but the lower edge …
It would limit the amount of area that could cause overlaps ( Which are a real pain to remove !!! )
I use Rhino at times as it has a function to select an edge and that can be used as a cutter to split any geometry that intersects with it…then it is just a matter of selecting the cut-off area and deleting it… works similar to Knife Project but far better without having it’s limitations …If only we had it in Blender…
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Solidify and then remesh will get rid of the overlapping geometry. You’ll need pretty high remesh resolution but if you have enough RAM and your computer doesn’t crash, you’ll be left with a manifold mesh
Sadly, this wont work as I add thickness of 1 mm and the inside geometry goes through the front lights. So the remesh will destroy the geometry.
That is ok for visual, but I need to prepare it for 3D print
Add a solidify modifier, use even thickness and a thickness clamp, that should prevent self intersections.
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it prevent but limits its thickness to the first intersection. So it wont let me add the thickness more than a fraction of a millimeter.
How about selecting all faces, extrude, then alt-scaling inward along the normals?
Make a new mesh that is much simpler than the current one, then offset that simplified mesh to be your interior surface. Now you just need to connect the perimeter edges.
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Well then based on how its calculating the thickness what you want is not possible. At its very basic logic the thickness is based on offsetting along the surface normal. And there is no way to prevent face overlaps with neighboring faces without restrictions for any case and for foreign faces its even worse. Volumetric methods could potentially easier prevent such overlaps but yeah its not like that.
So do what @SterlingRoth suggested,
or alternatively solidify it with overlaps, select problematic areas from inside, grow the selection a bit to be sure to include hidden faces and perform a mesh fairing in such areas. That will solve it aswell.
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Thought so. I would like to have the option as for example meshmixer has. That in the point of overlapping the polygons merge and continue to expand in the right direction as they are blocked do go through the mesh. Its calculated not by each polygon but much of a whole surface as it would remesh do that.
Yeah sure, I got your intention and that would definitely be nice to have, as in the end its just about as shape preservance and getting valid topology, but meshmixer is approaching it diffferently. And voxelbased meshes like openvdb remesher results are just a strict marching cube based outputs, so there is no such thing as a selfintersecting face there.