I tried that on a old top fuel Mopar cast iron block, most block filler are porous and lets the water flow right through it If Hard Block is the plastic epoxy filler it may not let water through, maybe it will
I ended buying some block sealer from Goodson that you had to put in the block under pressure for 10 to 20 minutes and then empty it, it was resuable and have the block or heads cook in a oven for a set time at a high temps, under 400 F if my memory is halfway near correct That was twenty years or more ago that block sealer flat worked, that block leaked like a siv before, not so after that sealer was done