Beginner questions about UV/texturing

I’ve just started to try out some texturing in cycles, and try to use the unwrap and UV tools. However, things are not working the way i’d want them to =)

I made a really simple image, that i want to use as a texture. Just want to add some lines/panels to the hull of the AT-AT im trying to model.

However, its very high-poly. Lots of small faces and edge-cuts all over the place.
So when i unwrap it, there’s a bazillion of tiny islands. I’ve tried to unwrap only the faces i want to texture, but no matter what i choose, it seems to unwrap every face of the object anyways? Am i doing it wrong, or is this working as intended? I’ve tried to mark the seems to keep them seperated from the rest, but, well…it still doesn’t really work.

And even if i try to move the faces away from the picture, it still seems to project onto them anyways.

So, how to do i “get rid” of the faces i dont want to texture? Sorry if this is a silly question, i’ve tried to watch some tutorials and searched for some tips, but i cant find anything