As to why you’re getting that pitting, I’m not certain. What a
pest when it happens, particularly when you’ve worked so hard to
solder that seam under difficult/risky conditions. What I
sometimes do (after an initial filing)is to use a rotary
burnisher in my flex-shaft and smear metal over the porosity.
Then just emery and polish as usual. Pits all gone.
You can make one of these gadgets from an old bur. Nip off the
head and grind the tip to an equal thickness with the shank. Hold
about 1/4" of the shank in a vise and use your torch to heat the
shank enough to bend it with pliers to about a 45 degree angle.
When it’s cool, take it out and grind/sand/polish the tip to a
mirrorbright dome. Put the new bit in your flex-shaft and it
will whomp a thin metal smear where you want it. Kind of like a
spinning hammer. (note: anyone with corrections or improvements
to my technique here, feel free to jump in)
Hope this helps.