Polishing inside small box

Greetings everyone - I am wondering how the heck one polishes inside
a small (less than 1") square box (with no lid). Is this one of those
jobs for an ultrasonic? I did a large number of solder jobs on it
with anti-flux on the inside seams each time, and now it’s a real
mess. Perhaps I just need to recharge my pickle?

As a side note, I’m using a very old bottle of anti-flux that says
it contains Boric Acid & Isopropyl Alcohol. Would I be guessing
correctly that this is rather toxic when heated? And I’ve been using
steel (they don’t actually say ‘stainless’ on the package…)
straight pins to hold parts while soldering, and pencil lead inside
hinges…am I doing a number on my lungs here with this stuff? I do
have an exhaust but only just yesterday clued in to the fact that my
soldering station was probably just a few inches too far from being
directly under the vent to be helping me as much as it should…

Thank you!
Jessica in a suddenly overcast and breezy San Francisco

Dear Jessica, What exactly is the problem with the soldered seams?
Where did you place the solder paillons to join the seams. . .inside
or on the outside of the box? Are you working with a texture on the
outside walls? Did you polish all of the walls, inside and out,
well before soldering? Why did you anti-flux the inside seams?
What grade of solder did you use. . .easy, medium, hard? How much
heat and how long did you heat your piece and in what stages did you
solder all of the walls? How many seams. . .5? What type of
polishing tools do you own to get inside that little area?

Will be glad to help out here with a little more info from you.

Happy finishing. . .a very time-consuming process:)

Kay Taylor