Why aluminum paint?

I've got a tin of cold galvanising paint that carries the following warning

"CAUTION care should always be taken when removing the lids of tins containing metallic paints as these are liable if contaminated (eg by moisture) to a build up of pressure during storage.

DO NOT SHAKE

Open carefully, preferably shielded under rags, especially if the tin has been allowed to stand part full. Apply with dry brush"

The tin is also the only one I have seen that came supplied with 2 spring clips to hold the lid in place.

Do you recall what it said? What type of warning was it? Mess or explosion?

I know people who have had smoldering fires from catalyzing oil finish rags, so it's not just hearsay. I dry mine on the sidewalk for a week before putting them in a trash can. Why take chances? When they're hard, they're safe. Wet and crumpled, they get hot to the touch. My first clue was picking up a hot rag in a cold shop. That brought the warning home to me.

My two favorite wood finishes, Watco and Waterlox, are both catalyzing oils and prone to that.

Credibility comes in different flavors. If a handyman club post was giving advice on some esoteric technical question, then skepticism would be warranted. But when the post is about 1st person this-happened-to-me, and the consequences weren't a matter of interpretation ("...coated the floor, me, my boots, everything in aluminum paint"), then I'm inclined to believe him. Why would he lie?

Bob

Just "Do not use to shake aluminum paint!" (in big red letters, IIRC. :-)

I've always believed the warnings I learned in elementary school science class, but I've not worked with those materials. I tend to also put oily rags in a metal trash can, not a plastic one. :-)

O.K. So the remaining question is:f

"How hot can a half-full can of this get if shaken?"

Enjoy, DoN.

OK. ;)

Ditto.

No idea. As I said, I always 'argon it' if I don't have any Bloxygen handy. (can of inert gas)

Totally different conditions and reaction than whatever's going on (if anything) in the can.

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_IFF_ (the proverbial "big if") this is actually occurring (of which I'm still not convinced there's not something else at play) I doubt it's actually much temperature related (at least from internally generated heat; the hypothesis of starting w/ something hot from exposure to sun or the like aside)at all but some sort of outgassing or the like that's generating the internal pressure rise with some modicum of additional heat perhaps. If this were such a major issue it seems the cans would have all sorts of warnings and it wouldn't need just a handwritten sign on the mixer at the local co-op.

On the tales of spontaneous combustion, HS chemistry teacher used a rag to wipe some spill from the lab counter after a demonstration in last class period of day class and hung the rag on a clothes hook and closed the door in a small closet. Next morning the door was heavily charred in the area and the rag was mostly ash on the floor. Fortunately, it appeared that lack of O2 in the small closet prevented full-fledged fire. He purposely never let maintenance repair the door as a reminder both to himself and students...was pretty effective teaching aid. I, also, to this day, am certain to not leave finishing rags in precarious places.

I posted the warning message printed on the lid of some cold galvanising paint I have, it would be difficult to miss when opening, on the 20th and it said not to shake and also that pressure build can arise due to moisture contamination. I wonder if the zinc reacts with the moisture to produce some zinc compound and hydrogen resulting in a pressure build up in the can.

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Perhaps but that's a completely different system than the Al paint of the subthread discussion...

I was thinking that reactions with aluminium are similar to zinc and apparently aluminium can react with water to produce hydrogen gas.