The newer mandated Propane canisters also have a valve which requires that you have something on it like your weed burner in order to get all the propane out... that did not used to be the case... you could just open the valve on the top to empty....
I did this the other day and took it to get it refilled... and when the guy weighed it he said it still had more than a lb in it...
So , from what yall said about propane/R12 contamination...... We should be careful to Power EVACUATE the RV canister before using it to catch any R12 we might need to save....