How to refill a spray can

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As usually, we (especially I) will not be responsible for any damage to you, your ego, your stuff, tools, house or anything else you own (or not own).
The things decsribed on this page may be potentially harmful to you and/or your environment (including other people and/or animals).
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06.08.2007: Warning: Some days after refilling the spray can with water, the sprayed water turned reddish orange - the can started to rust inside as I mentioned in the hints. Therefore, I can't consider this experiment safe if you keep the water-filled can around longer than some hours. The best way is to empty it out.

The Idea

Lately, I saw an empty hair spray can between all the other empty cans in our (you guess it) empy-can-box.
These spray cans are little pressure tanks and I thought there must be something more useful to do with them than to throw them away.

I emptied out the remaining propellant and asked myself how many pressure it could take. I came to the conclusion that the propellant must (mostly) destermine the working pressure. The ingredients list revealed me: dimethyl ether. I looked it up in the internet and found out that its steam pressure at 21 degrees Celsius is about 5.1*10^5Pa (=5.1bar). Well. Spray cans aren't always kept at 21°C, so we can expect a pressure resistance of at least 6*10^5Pa plus some safety margin which I expect the use at production. Please note, that the nominal pressure of a spray can depends on the ingredients and may vary, espacially if another one uses a different propellant.

First, I took off the spray nozzle of the spray can. After doing this, you can see a small pipe sitcking out. This small pipe is normally pressed in if you press onto the spray nozzle and opens the spray valve. To refill the thing, I took a normal PET bottle and drilled a hole into the bottom and the screw cap into which I inserted small hose parts. The hoses are hold in place and sealed with hot glue. The inner diameter of the hose is a little bit thinner than the outer diameter of the spray can pipe, so that it doesn't fall off. I filled a bit of water into the bottle (as much as I wanted to put into the spray can) and put the screw cap hose onto the spray can pipe. I didn't slip it completely over the pipe, because I needed to press it in to open the valve later on.

I hooked up the bottom hose to an air pump and pressurized the bottle to some safe 4.5*10^5Pa. (Most bottles can stand about 8*10^5Pa so 4.5*10^5Pa can be considered a safe value for both, the spray can and the bottle). Now I held the bottle vertically to make the water flow into the screw cap hose. After pressing the spray can pipe in, the water flows into the empty spray can. (Mine filled quite slowly, it took about 2 minutes to get 100ml water in). In between I had to pump it up simetimes because the whole construction wasn't completely tight.

After all the water is in, there's no need to hold it vertically any more, but I did so, because then one can hear the air bubbling into the water. After the bubbling has stopped, the can is pressurized to the same pressure as the bottle. I took the hose off, put the spray nozzle onto it again and now, well, I have a water spray can.

I've made a nice SVG-Drawing so that you know what I'm talking about here.

Some hints

Don't overpressurize the can this can be dangerous!
Don't fill in corrosive liquids as they might destroy the can from the inside and it may explode after some time. I don't know if even water is dangerous, because it could make the can rust.
Usually the propellant is some type of gas which liquidifies under moderate pressure (like the dimethyl ether). If you press on the cap and some gas is lost because you are spraying, there's still plenty inside which can vaporize. With air as propellant, the situation is a different as the air won't get liquid, leading to less propellant. I don't fill the cans more than one quarter with water (or whatever you want to spray) to have enough propellant inside to spray at a decent rate.
The nozzle won't vaporize the liquid that well because usually the propellant is mixed with the liquid (has to do with the previous point).
I think using old paint spray can't it not a very good idea unless you like a mess :-)

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