plumbing question

Plumbing question. I'm not Mr Handyman, but I'm not helpless, either, so please bear with me.
I have a very old house. All the plumbing is 1/2" galvanized lead pipe. I'm replacing a sink in the garage. The valves under the sink are brass, outdoor type. The lead pipe comes out of the wall, then the brass valve, then another short piece of lead pipe, then the flex hose to the sink faucet. I've done this type of thing many times before, but this time the pipe coming out of the valve just will not seal. I've replaced the valve itself. I've tried different pieces of short pipe. I've cleaned the threads. It just will not seal. I'm using gray pipe thread compound with the brush, but it's a few years old and has separated. I can stir it together, but I'm wondering if that may be why it's not sealing. The cold water side and the rest of the hot water side has sealed fine.
Should I go for an indoor-type valve? Should I get new thread compound? Is there a better type of compound or sealer that I should try? Any thoughts would be most appreciated. Thanks.
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Your supposed to mix the thread sealant right before use.
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The Dave wrote:

Correct. Galv iron.
Throw out the can of old dope. Buy a roll of Teflon tape for pipe seal.
Jim
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Correction. Probably not lead pipe, but it is 1/2" galvanized.
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I did, but it doesn't want to mix very well. I assumed that was maybe because it was old. When it was a new bottle it mixed fine.
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