What keeps the acid bath from coating the inner wall of the piece and eating it away from the inside out? Do you block/plug up the hole with something so no fluid gets inside the part?
Yes, plugging the ends of the tube is essential. If the part being etched is a 1" or 1.25" tube (for most of my inner G-Core chassis parts), I use the rubber cap that came with my pre-made blades to block the acid from leaking inside the inner wall. For MHS tubes, I need to find a rubber cap which is tight enough around the ~1.45" diameter. Until then, I use the clear packing tape with this method for a water tight seal:
1) Place a single piece of wide clear packing tape face down on the MHS's tube opening and press down firmly against a table top to set the tape.
2) Trim the excess all the way around leaving 1" of overhang.
3) Cut the overhang into strips every 30 degrees.
4) Fold those strips down against the outer wall of the part.
5) Wrap the outer wall with clear tape to double the seal of those strips.
6) Squeeze all air bubbles out with the rounded end of your hobby knife.
The benefit of the clear packing tape method to plug holes, though somewhat tedious, is that you can still see inside the tube. During etching, I'll check every few minutes to see if any acid has leaked in.