See my post below as gondar, similar subject. Try what I said. I have a 93 960 also. The silicate mentioned--grandmother called it "water glass"--does have sealing power and is present in some coolant, e.g. Mercedes. There are indications that enough of it to do much good is also abrasive to water pump seals.

I can't help with the porosity problem. I assumed it would be an early life problem. My 92 960 had a 21psi water pressure cap. I am presently running with no pressure (read loose cap) without any problems to reduce exposure to blown radiator . Some volvos have caps with about half this pressure which I plan to get with my next parts order. This might reduce your leakage until you "fix" it.

Forgive my ignorance, but does the porosity problem appear as hair line cracks?

Since you ARE loosing coolant-and are willing to try a sealant--here is what works for me: Go to your local auto supply store--get a bottle--or can--of a sealing product containing "sodium silicate"--used to be used as "egg preservative"--really--also used to be the auto industry did not really know about its' great "sealing" properties for leaks like yours. My college auto mechanics professor was "old school"--we used to buy the sodium silicate AS egg preservative to seal leaks.
Follow the directions on the container. Basically, what we would do, is first drain all the coolant out--flush with clear water--remove the thermostat. Open your heater-fill the system with water only--leave room for the sealer. Close up the cap--idle the engine until VERY hot-we would literally block the radiator to restrict the air flow to achieve a good high temperature--however--do NOT get into the "red zone"--or if you have a light--the instant the red comes on--shut it down. Leave it be 24 hours--overnight-- until it THOUROUGHLY cools down. Start the engine--watch for the leak--it should be sealed. Drain the water--put in your anti-freeeze and fill 'er up.
IF you can still find "egg preservative"--Sodium Silicate-- at some place other than an auto parts place-go for it. I think we used to get it at a "poultry supply place"--been awhile--might be available at a pharmacy--donno--things change-