What is the right way to draw transparent 3D Models from collada file?

I am trying to load a model via assimpModelLoader and give it a bit of transparency via ofSetColor().
No matter what object or format i use I have strange glitches in the opaqueness.

Is this a depth problem? or ofSetColor() not the right way? or is it the light?
or is this just not the way a mesh works?
I am sure i can’t be the only one trying to load and display “glassy” objects :wink:
and just need a hint in the right direction.

(it’s supposed to be all smooth (no cracks) and with a bit of lighting shadow maybe)

i used the assimpl osx example loadedMy collada object and set the color via ofSetColor() before drawing.
(but the problem appears in all models not just in mine)