Flush a radiator out of a truck?

[HotRodScottsdale]

So I got a used radiator out of a parts truck that I got for cheap. The truck had a blown head gasket and the guy used fix a leak in the radiator to fix it, he said it worked for a year and seemed to be running fine when I got the parts truck. 3 years later now I pulled the radiator from storage and wanted to use it but it's still gummed up with that fix a leak crap. How can I flush it without getting that in my motor. Tried a hose and got nothing.