If you purchased the Por15 reducer, you might get lucky and have an opened can last one month. Hopefully you purchased the tiny cans. Here's how I do it when I'm not using it all at once:
1. Do not shake can prior to opening.
2. Open lid, stir gently (with something clean and disposable like a plastic knife) and remove a small amount (I use a turkey baster) into a sepperate container (I use the disposable plastic storage containers sold in the sandwich bag section of the supermarket).
3. Clean lip of lid and can thoroughly with a Por15 reducer dampened rag. (If you don't do this, you'll never get the can open again) Some people use a platic bag between the lid and can.
4. Pour a small amount of the Por15 reducer in the can, just enough to cover the top of the Por15. (do not stir)
5. Re-seal can tightly and store without shaking can.
Note:
Do not put brush into can of Por15.
Do not use reducers other than that available through Por15.