Just as there are different types of engines (alum, cast iron) there are
different types of leak sealers.

1. Bars/prestone types
2. Aluma-seal types
3. Moroso ceramic types

1. Bars/prestone. These are almost totally organic (no metals) and work by two
methods: Seal swelling & gluing. There are some solvents in it that make rubber
parts (like our water jacket gaskets) swell slightly and seal better. Also works
on connections between hoses.

Then whats left of this stuff basically coats and hardens in cracks and
crevices. This might help on very minor cracks in heads/blocks. This is where
bad things can happen. If it coats and hardens in radiator or heater cores it
can permanently clog them up. But if it seals the joint between the head and
block/gasket then its good.

2. Aluma-seal types. These have a mixture of metal fillings and a binder in a
powder form. The filings give strength to any chunks of it that forms a plug at
a leak. The metal does not melt, the engine/coolant is not hot enough. This is
best suited to sealing cracks in cast iron parts and may not be as effective as
the others on our typical rubber water gasket problems.

3. Moroso ceramic. These are newer then the others and harder to use. You put
the stuff in, run the engine and then let it sit to harden any that was
deposited in the cracks/leaks, then I think you drain and fill clean coolant.
While these may not have any effect on rubber parts it will get deposited in the
crack between the head/water jacket/gasket and seal there. It should also do a
good job of filling up typical pitted areas on the heads.

Most of our successes have been with #1 and #3.

There are many books/manuals that warn about using MORE then one application of
any of these products. The directions on Bars says "use once a year as
preventative maintenance" , "lubes the water pump too"

I have used Prestone sealer and it does work. Got about 80,000+ miles on one
application.