Object seems OK in EDIT and Object Mode, But when I try to 3d print or remash its all messed up

Hello guys…
Im a beginner with blender. Im current trying to model a 3d print pen support, I used some operatios during the proccess like Boolean, spin, bevels…Its a simply object…But I just cant make it work.

Heres is the deal: I had already finished the object. When I see it in the object mode or the edit mode, the whole model seems OK. But when I try to slice inside my 3d printer slicer, or even remash, the whole object appears the add an another layer of ground in the middle, or something like this…Those places that are supposed to be hollow

when im trying to remash or 3d print:

*filled spaces that are supposed to be hollow"

(I can only post 1 image… That sucks…)

I already tried to fix normals, merge by distance…( I actually used all fixing tools available)… But nothing seems to work…ITs always filling those spaces with this layer.

like I said, im beginning with blender, so any help will be welcome.

Thanks!

You’d have to post the .blend file so that we can see what’s wrong with it.

That’s easy enough to fix: see the FAQ