Don't open the can.

Use a rivet for the hole to pour out what you need, then put the rivet in to seal the hole. Rivet works best because when done you pop the rivet and seal the hole. tip the can upside down once and back upright and that seals the inside of the rivet. Use a new rivet and new hole. I also put the can in a plastic bag and get all the air out that you can. It keep it a little longer.

hope this helps