That’s an old trick and a good one. I’d like to add a few pointers,
if you don’t mind. When you do this rotary burnishing technique, use
an old number thiry hand piece. If you use your nice quick change,
you’ll beat the bearings to death over time. You can also use a
hammer handpiece with a straight bit with a rounded and polished end
and just planish back and forth across the porosity.
Also, I’ve found the after burnishing, rubber abraisive wheels tend
to pull up the porosity all over again, as they contour themselves
around the hard areas and cut down into the spongy places. Better
results from going through finer and finer sand paper, ending with at
least 600 grit, then a quick hit with tripoli and finally rouge.
You’re right, stay on it too long and hard, the porosity comes back.
Finally, don’t bother trying to fill porosity with solder, it’s a
waste of solder as it can’t really wet properly over a porous area
because of escaping gasses. Maybe after you burnish it down, if
you’ve sealed it up well, you can fill a small low area with solder,
or, of course, laser weld.
Finally, it’s always a good idea to see if you can learn why there’s
porosity there in the first place. Mold too hot? Improper sprueing?
Contaminated metal? Abrupt changes in mold thickness? Look at the
pososity under a loupe. Is it from gasses or shinkage? Gas porosity
looks like tiny pinholes. Shrinkage leave spongy areas, often near
the sprue or where a thin area is next to a thicker one.
David L. Huffman