SpudFiles

What you should first do is use a wire wheel to clean off as much of the compound as you can. Then, soak the threads, inside and out, with a penetrating oil such as Kroil (GOOOOOD stuff) or PB Blaster. Let it soak overnight, dribbling a few more drops on every few hours. You don't need to splash it all over the place, just 5 or 6 drops on the threads. Anything else will run off and do you no good. After it's soaked overnight, stick it in a bucket of ice water. Let it soak for about 10 or 15 minutes to get nice and cold. While it's soaking, put a pot of water on to boil. Once it's soaked for a while, dip it in the boiling water for about 30 to 45 seconds, just long enough to warm up the valve, while hopefully keeping the pipe somewhat cool. Then, immediatley put the pipe wrenches on it, and see if it breaks free.