if it’s so porous, have you tried contacting your caster? perhaps
you need to talk to them about redoing a bad job? You’re paying for
quality results and you didn’t get them. Such results can come from a
few things, but one common thing would be using too much “old” metal
when they cast, and considering the current costs of metals, I
wouldn’t be surprised if some casters are tempted to use more scrap
than they would usually would(you should never use more than 50%
‘old’ metal, and that needs to be pretty clean as far as solder etc).
If you do need to fill a hole in a casting, what I have found is that
you can drill out the hole and clean it up. Then take a small ball of
silver (melt a small piece of scrap) and some solder and place it in
the hole and heat it until the solder flows in and around the
ball… then file it down again. This only works with larger holes.
With some of my castings, I get internal air bubbles in the wax that
become holes when I cast. I don’t always see them before it’s cast,
so that is how I fix it. When I try to use just solder, it doesn’t
flow into the hole properly. But if your piece is porous in general,
I’d complain to your caster.