Hi Bob, I don't see the Loft that you mentioned in your attached file - how are you constructing that loft, is it in some individual surfaces kind of piece by piece?

If so then you would want to use the Edit > Join command to glue those pieces together into a solid.

Boolean union is meant to be used more for cases where 2 objects are kind of pushing through each other and some material has to be cut away from them.

If you have surfaces that touch each other edge-to-edge then it's often better to use the Join command for that (particularly if you've got a bunch of individual surface pieces, since booleans are also focused on working on volumes as well) - the Join command is focused on gluing together surfaces that touch at common edges and does not try to slice away any material like booleans do.

But in order to use the Join command the edges that you want to be join need to be unattached to other surfaces.

So for example in your case here there are some end caps on the ends of the sweeps, like this one here highlighted in yellow:



While that end cap is in place there you won't be able to join the side sweep surfaces to some connecting loft surfaces because the side sweep surfaces are already joined to that end cap. So you would want to remove that end cap (just select it and push Delete to delete it) so that the end was totally open and looked like this:



With those edges all open like that you can then use Join to glue some connecting loft surfaces to them, and once you have glued all open edges together you will then have a solid.

If you're still running into problems, could you please post a model version that has the loft surfaces in it as well so I can see what kind of connecting loft you are constructing?

- Michael