And for those of you who are not familiar with blind end inserts, here is a picture of one on the left-side next to an open ended insert on the right-side. Note the solid bottom on the blind end insert.
Now this is a picture of a brand new blind end insert, before being sanded and then etched with muriatic acid. The bottom is easy to sand, but in between the threads is not, that is why I use the muriatic acid since it gets everywhere. Just be careful, muriatic acid will give off a visible cloud of acid when bottle is first opened, do not want to breath or get in eyes, so I wear safety glasses and wave my hand around to disperse the initial cloud and any after pouring into a small plastic container, then cover container so no more fumes are released.

And here is the setup I use to treat blind insert with muriatic acid, cover is off. Use two nuts to lock insert to bolt and screw into fuselage, then un-tighten nuts to release bolt from insert.