Ah. Didn’t use a lot of tools in the video, it’s more complicated to understand the mesh errors that get selected with select non-manifold (ctrl+alt+shift+M, or from the select menu). That also has options on the operator panel to limit the error types it detects. Still, probably would need to explain the steps and reasons behind them for it to be more clear.
I’ll defenestrate the screencast keys addon which isn’t helping. It’s too busy, showing every key press. Removing navigation, it’s just the mentioned mesh selections, deleting mesh elements, filling faces, and remove doubles that gets rid of the errors.
I’ll attach the end result from the video. Might not be modeled how you want it, but there are no mesh errors left at least, which was the requirement in the question.
Intersecting geometry like with the leg parts is not a mesh error. It’s normal that a model consist of multiple mesh parts, intersecting or not. When all parts are manifold, or “watertight”, it’s error free. Open edges are also normal error to leave in the model, usually left because faces from the intersecting parts that are not seen can be removed. But it all depends on the requirements that come from the end use.
Don’t know what it’s for or why it needs to be “watertight”. If it’s for 3D printing, the model is not ready for that. Like I mentioned, intersecting geometry might not be a problem for that if the importing application can interpret those (such as Slic3r), but some parts are apart from the main model or just barely overlapping. If it doesn’t accept intersecting geometry, or you want a hollow model to save material and time, the parts need to be combined to have one surface. That way it’s easier to describe a hollow model because the backing surface can be added with solidify modifier for example.
Many ways to combine the parts. Not a big fan of boolean operations, but it’s one option:
I selected something from the two parts that need to be combined. Then ctrl+L to select everything linked to those and hid everything else with shift+H. Then I selected one of the two parts with the same way, and ctrl+F -> intersect (boolean), and changed the operation to Union from the operator panel.
That combines the surfaces, but the resulting structure is pretty much crap, like expected. Would need to merge vertices, cut new edges, to clean it up. Or if it’s for 3D printing and it triangulates properly, could leave it as is, printing doesn’t care about pretty structures.
Alt+H to unhide the rest.
Mech_ja12.blend (123 KB)