2002 Honda Civic overflow tank overflowing

Someone in my family had a problem like this about 6 months ago with their `98 Chevrolet Malibu. There was a lot of sludge building up in the reservoir and it seemed to be restricting the coolant flow out of the reservoir.

This eventually lead to a pressure buildup which in fact blew a head gasket, spewing scalding coolant out from under the hood (looked like the car threw up!)

A radiator flush did not help it; came back just a few weeks later.