Your Helmet has like 7 floating shells apart from the main one. Tools and functions will fail if there are separated shells or nonmanifold (intersecting/overlapping) meshes. I would not use made solid for either of these models. So is more acceptable for organic models rather than something detailed made from brep. That tool I mentioned if useful for for really bad scans and instead of spending 2 hours clearing garbage and manual bridging connections it tries to rebuild from scratch a watertight model, but it looses a lot of detail.
