I think the ball in the paint is usually a marble or a ball bearing - whatever is cheapest I'm sure. The ball is to knock the pigment off the bottom of the can so it will mix. Like the pigment stuck to the bottom of a old paint 4l can that you scrape off and mix into the liquid.I've often wondered why a ball is even used unless it lends itself to can insertion. Its not really a great mixer-upper shape, especially gas & liquid. My own experience is the solvent & paint mix & then its diminishing returns. A lot of guys swear that moderate heat is better, softer spray or better build. But paints in spray bombs vary all over the map, who knows. I have heard there could also be some kind of valve effect inside a chamber but doesn't seem clear. Also turns out balls can also be made of glass so excessive mechanical shaking could result in adverse effects?
What is the rattling sound inside a spray can? So many curious people, such as I, wondered that for so long. After I emptied a spray can after painting some models, I remembered this as one of the things I am curious about. So, I decided to open an empty spray can to find out.thediecastmodel.com