I'm afraid that my explanation yesterday may have done more to confuse than inform. I will address this in the following post. Now, I will tell you why this model can't be turned into a 'solid'. I made this model for display only; for example, to render as you did. This model will be changed for 3D printing and casting. For example, if you look, you will see that the screw holes have no bottoms! Obviously, that won't make a solid. I should have told you this; I'm sorry that you wasted time on it. Also, on the model to be printed, I have to add material to all the surfaces that will be machined after casting. This model is just to look at.