The loctite part # 17497 - 1 Litre loctite 990 IMPREGNATION SEALANT.
A nice description I read was:
"Years ago new castings were pressure tested for porosity prior to machine work was started. Porous castings were junked prior to machining. Today, porosities are repaired using Loctite 990 which is surface applied and cures at room temperatures. 990 can seal a porosity to the pressure rating of the parent metal. Factory castings are dipped and cured using Loctite Resinol wicking type porosity stop. One type cures at 194F degrees and one cures at room temperature. The porosities are rectified for a lifetime. Aluminum is the most prone to porosity."
I can confirm that this product is fantastic. A friend of mine used it on his 1967 P400 Miura and not a drop of oil comes through the castings anymore.
I'm not sure if Loctite still makes this stuff, but Hernon makes a similar product that shows that it is equivalent to Loctite 990. Their product is called: HPS 990. I notice they also have higher rated 1000 and 1100 version as well.
I also would like to know where you put it onto the block, inside/outside or both?
My valve cover leaks through the Aluminium and I'd like to seal it, if anybody could organize one of these two 990 products I'd be happy.
I found the best way to apply is to gently heat the casting with a MAPP gas torch (an oven will also work nicely). I poured some into the internal water & oil passages, drained & used the residual to coat the outer surfaces.