Are you segmenting from a CT volume or have you just imported an STL model?
If you are segmenting from a CT volume then in segment editor you can use the smoothing effect with the smoothing method set to “Closing (fill holes)”. Try different sized kernels, the bigger the kernel, the more holes are filled. Just be careful not to have it so high that you fill too many foramen holes/other anatomical holes which are supposed to be there. Depending on how patient you are then you can also manually paint in areas of sketchy bone in the 2D slice windows.
If you are worried about there being lots of holes around the orbital walls and the sinus bones then this is a more difficult problem to solve. These bones are notoriously difficult to segment because they are so thin (down to around 0.5mm thick) their hounsfield units are very susceptible to being under-represented due to partial volume effects.
To get the best results you need a scan which has small voxels i.e. small field of view and small slice thickness. Then you can use the instructions on this post to get a slightly better segmentation: Enhancing orbital walls with Unsharp mask filtering
In the past if I want a skull model with nicely defined orbital walls then I first segment the skull from the main volume. Then I will crop out a volume for each orbit using two ROIs, then perform the unsharp mask image filter on these volumes, segment the orbital bones in separate segments, then combine these segments back into the original segment. This is easier than performing unsharp mask filtering on the whole original volume because the filter does have some negative side effects such as rasing the HU units of the skin etc.