Most metals are slightly porus.
If its a copper h/s, melting solder into the base would fix it, and the exess would come off easly when lapped. I used solder like this to seal some porus braizing on a leaking barb on one of my blocks . Would have been better if I could braize properly though...
In many industry using sintered metal products, they are
"infiltrated" after with a copper alloy to seal it. Requires furnace with inert gas atmosphere to use anything other than soft solder though....
Dont know what filling the porus holes would do to performance though. I think it would be a small improvement, but noticable? probably not.