I can't answer your question and I fully appreciate your concerns. I do have several concerns of my own:

1) Leaving a gas can open to allow vapor to evaporate is terrible fo rthe environment. Sure, if you are the only one doing smog won't go up in your city, but still we each need to care about the environment. Volatile Organic Compounds such as this are a major contributor to smog - this is why automobiles have close tanks with vapor recovery and why many parts of the country have special hoses on gas pumps to capture vapors released during fueling.

2) Washing out the cans has much the same problems. What happens to the wash water.

3)IF (and I know this is a big if) the cans are properly sealed it should not matter if they are full of the more dangerous vapor versus being filled with gas.

Maybe store them outside under a tarp during the winter?

Best of luck finding a solution to address your concerns.

There is no known way to safely handle gas cans, it's a problem, we should not be using gasoline (the tree huggers say so).
BTW, gasoline today will stay fresh for several months - I personally would be more worried that some kid would get into a can stored outside under a tarp.
My thought's keep the can lid on tight, in the garage, away from the open flame of the kerosene heater(open flame of kerosene heater, enviromental friendly??) what do think wheelhorse?
There is nothing safe anymore. LOL

IMHO,

-- Empty the gas into my car. I would rather store an empty can than a full one.

-- Put the lid on and store it the greatest distance from the heater as possible

lmy

I agree that there is not a 100% safe way to handle/store gasoline (or other solvent type VOC liquids). An approved flammable liquid cabinet made of steel is required in commercial applications already (look for California to require it for home owners, with other states to follow thereafter). If you need peace of mind, you might construct (or purchase) a metal storage locker (with provisions for padlock) and secure that to a place outdoors, under a roof. You could secure it inside a shed and provide vents "through the wall". These are only suggestions or ideas and I don't imply that they are the only solutions. I personally store any remaining gasoline (and I try not to have much left over at season's end) in the plastic gas container in a lockable shed with my tractor and mowers. I don't use the shed for anything but storage and it is not heated. It is also vell ventilated (due to gaps in wall boards and screened eaves). Because I add stabil to the gas container WHEN I BUY my gas, the leftover fuel is fit to store without me having to remember to add it to the last tank of the year (who can predict which one that would be?).

Hmmm... I had planned on emptying the gas into my car. Maybe I'll give the empty cans out as Halloween premium treats. Just kidding, just kidding.

I guess I'll just sit the empty cans outside whenever I light the kero heater.

I say, bring the gas back to the gas station and ask for a full refund. Bring the container to your locak hazchem depot for them to clean it out properly. Then when you get home (on your bicycle of course) go out in your back yard and plant 20 trees...

..... yes I am joking!

Well, you could empty all the gas completly out, pour in a small amount of kerosene, swirl it about a bit, and coat the inside of the can: then cap it tight. Kerosene vapors are not going to blow your garage up. When you fill it back up with gas I wouldn't even worry about the kerosene that might be left in it. I remember back when times were tight my older brother mixed gas and kerosene 50/50, more or less, and ran it in a 1947 Hudson. Made it a little hard to start but once it warmed up it ran ok. Fortunatly we lived at the top of a long hill and on cold mornings he'd just roll it off the hill to start it up.

Ah, those the good old days.

>> ..... yes I am joking!

Well, I do have to admit that I got to wondering how much vapor I would be venting to air if I opened the empty cans and let them air out compared to, say, what is spilled from fill ups at any given gas station in a day but hey, we all have to do our part I guess....

Even if you air out the cans, store them with the caps on tight to prevent dust & contaminants from getting in the cans.
The problem with the kero heater would only occur if the cans sprung a leak. Something that has happened to me.
Also many years ago a friend had emptied & air dried a small gas tank to a mini bike in order to solder a leaking seam. It exploded as he was using the torch on it. Nobody got hurt, but the tank was all puffed out. Just a trip down memory lane.