I'm trying to install a new dishwasher, replacing the 20 year old one I currently have. I have a compression shutoff valve underneath the sink for the hot water water. It has two outputs, one going up to the faucet and the other going to the dishwasher. It's connected using rigid copper pipe. It seems I'm supposed to remove the copper pipe going to the dishwasher and install a stainless steel sheathed flexible tube instead. The problem is that I cannot seem to loosen the actual nut holding the rigid copper pipe to the dual outlet compression valve. It's green with oxidation, i'm assuming from the copper, and it seems to have some rigid white paste beneath the nut. I'm assuming this is pipe dope? I feel like if I were to turn any harder I could damage the thing. Any tips on a better way to get this loose. Am I missing something obvious?