This is a great video someone sent me awhile back - explains a lot.

If you're going to run a sealer through the block, dont use a cheap *** option thats just going to cause more issues down the road.
There are higher quality products for this situation.

This, for instance, has some pretty solid reviews and is not a glass based sealer - Head Gasket Repair by Dura-Seal® Sealers & Oil Stop Leaks

Moroso Ceramic Engine sealant - is a product a Rover resto shop uses for these situations as well in combination with top hat sleeves where necessary.