That must be a feature of the upgraded version 4.01 it is solid as can be depending of course on the size of the extrusion width and the model.. big nozzle little space can = holes... The other question I had for the OP is now many shells do you have? He says light looks like stars.. that is either very very under extruded or there are no shells .. I have a box with 2 walls (.8mm walls) that has one tiny leak in the bottom corner other than that water tight.. it was not made to holding water but it does.. almost ;-)
Back to the 100 infill I make a part I sell that printed 100% infill and i can tell you it is solid and the infill layers an not different than the top and bottom layers.. if there is a visual difference when you are printing, you may have the infill extrusion width not set to 100% or the infill overlap not set to 15-25%