Polishing the shaft

If one looks at the inside of a brand new TetherSpout through a magnifier, while shining a light from the other end, a lack of polish, including significant mill marks, is readily visible:

That is actually rather common in such small bores, and even my Male Chastity Now and Mature Metal chastity devices exhibit a lack of smoothness inside their locking sleeves. Industrial polishing of stainless steel objects is typically performed by a tumbling process with a mix of agitation media and fine abrasive, for instance ceramic beads and polishing paste. This works great on outside surfaces, but does hardly anything for bores that are too small to admit enough agitation media.
The lesser finish of inside surfaces is often a non-issue anyway. For instance, the above-mentioned locking sleeves are rarely exposed to a harsh environment, and ultra smooth metal-on-metal sliding is not an important characteristic. However, when a TetherSpout is used in a chastity system, it may stay in place in the wet urethra for many days or weeks at a time. In that context, the mill marks visible above could create a bacteria-friendly environment, as well as promote instances of pitting corrosion.

After noticing dark residue when wiping the inside of a TetherSpout during a cleanup following multiple days of wear, I decided to get rid of those mill marks and, if possible, polish the bore to a mirror finish. After looking high and low for ready-made and small enough bore smoothing tools and having no success, I came up with the following idea...

I use a 3/32" diameter, 2" long, cotter pin and squish the end loop in a small vise (while holding the body in a pair of pliers to prevent it from opening up too much) so the pin can fit through the spout's bore (and through the cross holes to be able to polish those as well). Here is a before-and-after picture of the cotter pin:

Then, I cut small strips of sandpaper, 1.5" by 3/4", that I pinch in the cotter pin like so:

And I squeeze the two ends of the cotter pin and place them into the 3/32" collet of a handheld rotary tool, to create a one-sided "flapper":

The tool in the above photo is a Dremel 4000, but the first time I tried this approach, I used a very cheap rotary tool I had bought at Harbor Freight Tools and it did the job just fine.

Turning the spindle by hand, I wrap the sandpaper tightly around the cotter pin, so I can slide the TetherSpout over it:

Then, while holding the spout between thumb and index finger, I turn the rotary tool on and I slowly move the spout back and forth (letting it also slightly rotate now and then to even out abrasive pressure), keeping all parts of it over the length of sand paper. My Dremel has a variable speed, and I usually select 10000 RPM. That performs the work quickly enough without the spout becoming uncomfortably warm. The cheaper rotary tool I used before had a top speed of 16000 RPM, but it did not go anywhere that fast under load.

A couple of points worth noting:

• Never let go of the spout while the spindle is rotating! If you did, the assembly would immediately become unbalanced and the mass of the wobbling spout would bend the cotter pin, causing the spout to be ejected and become a projectile faster than you can say your favorite expletive!
• I avoid letting the spout ride over the tip of the cotter pin while it is rotating. This could cause extra scratches inside the spout. I only slide the spout off when the spindle is fully or almost stopped.
• This process creates some fine steel and abrasive grime that sticks to the fingers holding the spout. Unless you have suitable hand cleaner (such as the "Fast Orange" product commonly used by mechanics), I would recommend wearing a thin disposable glove.
• When the sandpaper eventually gets fouled by steel dust, I remove the spout and get some of the grime off  the sandpaper by lightly slapping it. When it is really worn out, I remove the cotter pin from the collet. If I haven't already done so, I get another run with that piece of sandpaper by turning it around and pinching the worn side in the cotter pin, allowing use of the side that was previously wrapped-over and is still in good shape.
• I do an equal number of passes with the spout placed on the spindle in one direction and the other (e.g. sliding the spout on flange first, as in the above picture, and doing the next pass with the spout reversed, that is, with its tapered end on the collet side).
• Between passes, I blow into the spout to get rid of remaining dust.

I start with 220 grit sandpaper and only switch to a finer grit once I have removed all the visible grooves from the spout's bore.
I select sandpaper that has as flexible a backing as possible, so it can be wound tightly without cracking (for the 220 grit, I found 3M brand sandpaper at Home Depot with a plastic film backing that is extremely flexible).
Also, each time I am about to switch to finer grit, I do a pass inside the cross holes at the tapered end of the spout to polish them as well (they don't need anywhere as much work as the main bore).

In the case of the spout pictured in this post, here is what it looked like after wearing out two strips of 220 grit:

The majority of the mill marks were gone, but there were a few deeper grooves remaining. A couple more strips of 220 grit got me to that point:

And finally, having used up 6 of my little 220 grit strips, I was ready to switch to a finer sandpaper as all the grooves were gone:

Now, for the finishing, I used successively one strip each of 400, 800, and finally 2000 grit sandpaper, to get this result:

 

When I have reached that point, I can't really see any defects, but for the sake of perfection, I use a finishing kit with a buffing paste rated at 8000 grit, that I bought at a hobby shop:

The included felt pipe brushes fit just fine inside the spout and can be held in the rotary tool's 3/32" collet.

Finally, I wash the spout thoroughly. To remove any grime and buffing paste remnants, I pass a Q-tip through the bore (I remove some cotton from the end I push in so it can fit). Then I blow through the spout and rinse it some more, inspecting it to make sure no cotton strands were left inside.

And that's it, my new shiny TetherSpout is ready for use!