My hardner is yellowing in its jugs. The 2100f is twice as yello as 2100s. I keep it in a cabinet in my garage with temps 80-90*F. I am assuming this is moisture contamination and probably no good anymore. Are there any other possibilities?
Its likely old. Also when a bottel sits half empty for an extended period of time it will do that from the ait in the bottle. Try warming it up in a hot tub of water.