That's also what happens to a poorly maintained coolant system. Bet you've been adding water for a while. I took out the plug for the coolant drain in the block(mid way down block, around cyl.6, exhaust side), and hosed it out with a high pressure nozzle. Started at the front of the block, and worked my way back through every coolant port in the block deck. Mine wasn't really bad, but there was some rust, scaling, and block seal in there. Went back and forth until only clean water ran out. That was 2 years ago, plus. Changed the water pump today, and flushed out the cooling system while I was there. Nothing but clean water and coolant came out.
The BEST way, is to pull the block, strip it down, and take it to a machine shop and have it cleaned there. They can stick it in a hot tank, and it'll come out looking almost new.
