Leaky shut-off handle - replace washer or use teflon valve packing string?

There is a very slow leak from the handle of one of our water shutoff valves in the basement.
I was able to almost completely stop it by really tightening the packing nut but it seems that the real root cause is probably that the washer has gone bad.
At Home Depot, they only carried teflon string which the guy claimed you would use instead of a washer. Alternatively, I could go to a plumbing supply house and track down the right replacement washer.
Questions: - Is the teflon string as good as a replacement washer or should I try to track down the right washer?
- Is the teflon string really used in place of the washer or in addition to it?
Thanks
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A packing leak (water leaking out the valve around the shaft) is not the same as a valve leak (leaking of flow that should be controlled by the valve).
Replacing the packing with a "pre-formed" bit of packing or the use of "string" packing will solve a shaft / packing leak.
When you talk about a "washer" do you mean pre-formed packing or the "washer" that goes against the vavle seat?
Different leak paths in a valve need different fixes.
cheers Bob
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Packing leak
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open valve all the way, then try tightening packing nut again.
chances are good it will not leak
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