STL file fails when 3d printing

I scanned a car fender. I’ve sliced it into smaller parts then using patch and 2 rails made surfaces then extruded the surfaces by 3mm . The parts look good when rendered but when I attempt the print the STL files, I often get failures in some of the parts but not all of them. I repeated the process several times but still get failed prints .
Any tips ?

It’s better if you can upload the model and specify what slicing software & 3d printer you are using.

‘Looking good when rendered’ is not a good enough gauge of 3D printable-ness. You need to check if your solids are valid and watertight. 3D printing does not use the Rhino NURBS surfaces themselves, it uses a mesh generated from them. If you export STL without checking first, you have no idea if they are good or not.

If the Rhino model is good and closed, then there is a very good chance your stl’s will also be good. If not, there is no chance. But the best way to go is to first mesh the objects in Rhino and check the results, then export the meshes if they are good. If not, either fix the objects until the meshes are good, or fix the meshes directly, then export.

Some 3D printer software can automatically repair minor errors in stl’s. But I wouldn’t rely on it, I would always try to make sure the stl’s out of Rhino were good in the first place.