Seams can be soldered completely closed, and further soldering is
safe provided the item is never dunked in water or pickle until all
soldering is completed. The problem is not air expanding inside the
hollow piece, the real problem is traces of moisture entering the
hollow cavity through a pinhole or gap in the seam. This can happen
between soldering steps, or years later after being worn as
jewellery. Hollow items are often made of very thin metal and
cleaning up a soldered seam may expose tiny gaps.
Heating a completely sealed hollow item is always risky. A smear of
moisture inside will instantly turn into superheated steam and
things will go ‘POP’. Either heat very slowly and cautiously looking
for a tiny hissing jet, or drill a breather hole first and then
solder it closed again when the job is done.
Problems with having a permanent breather hole: It is difficult to
make sure the inside is free of acid and completely dry after
pickling and washing; It is un-hygenic because dirt and moisture can
fester inside the cavity, specially if the breather hole lies
against the skin.