why is it that when i have welded lines on my 3d object (supposedly solid), and i delete them to show a smooth one piece object, suddenly the object becomes hollow
Show a screenshot at least but I suspect the edges you are deleting are required to support the faces and create the surface. If that’s the case, deleting those edges is the wrong thing to do. Instead you should be softening/smoothing or maybe hiding the edges you don’t want to see.
Actually 3D objects in SketchUp are always hollow. SketchUp is a face modeler, not a solid modeler.