I've got a reverse-flushing adapter with a rubber expansion plug that fits the lower radiator hose nipple and allows me to goose a shot of compressed air into the outlet of the radiator after it's filled with water. A piece of rag wrapped around an air blow gun will do the same job, but it's just not as professional-looking! The radiator cap needs to be removed for this procedure. A quick shot of air sends the water backwards through the core, and dislodges most of the crud that's clogging the tubes. It can also open up pinhole leaks that have been stopped up by the deposits, however. Make sure to put a deflector around the radiator filler neck, or you'll flood the engine compartment, and possibly yourself, with dirty water! Stay far, far away from the powder-type sealers. As you've already discovered, they're just as good at clogging up cooling passages as they are for fixing leaks. Once the radiator is pretty clean, a bottle of Bar's Leaks would be the only sealer I'd consider using. It's a mixture of vegetable fiber and water-soluble glue.
Jerry
