Can spray-paint be stored flat?

Yes, you can lay the cans on their sides.

Yes, as always, you just want to make sure you shake it thoroughly before trying to use it.

Hi Thea, the only thing to remember when storing it on its side is that it can leak out of the nozzle. If you lay them at even the slightest incline, you will keep them from leaking.

Should not matter. You main concern is storing in a hot area or a garage that gets too cold.

Yes, I do....just make sure you have the cap on well so the nozzle doesn't hit anything while it is horizontal and spray all over.

Here's a post that might help - https://www.hunker.com/13416079/how-to-store-spray-paint-in-a-garage

If you keep the cap on it, you should be okay. Also, you will have to put in some type of area where it won't roll around. Perhaps a bin or block on each side.