Thanks for the respons! I did spend a lot of time making this indeed. Ill make sure to check out the website you posted in your comment!
Before I got your answer, I made some changes to the model: I spend the night correcting the 'red surfaces' in the upper model: Every red surface is gone by now and when I put it in 'layer-mode', no holes can be seen.
Though, I encountered a new problem: when I go through 'layer-mode', the model fills itself up when the inside should be hollow.
Any idea how to fix that? I put the new STL file in attachments.
Thanks already!
DeJerre
Test onderkant vol.stl