Uploading an object from Blender to UE4 looks different so I have created this ak-47 mesh using blender


and when I try to export it to UE4 it looks like this(I exported it as a .fbx file so I don't think this is the problem)

As you can see for some reason the parts that are white in blender works fine but the parts that are grey are not.

when I crated this ak I first built it as a one object and then separated it to 4 different objects so I would be able to add different materials to each part later and I think this might be the problem somehow.

I'm new to blender and this is the first time I try to crate something without a tutorial so this might be a very stupid problem but I couldn't figure it out so thank you for the help :)
I experienced this same problem. Open Blender and go into edit mode by clicking Tab. Then you need to select all the faces, and use Ctrl-N to recalculate the faces' normals. Finally click that thing that looks like a wrench and click solidify. By doing that you're making every face solid like a room and that should work.