I use Hematite beads from Hobby Lobby
Hematite is "already corroded" in the chemical make-up, so I don't worry about copper or steel bb's corroding
they also don't have the sharp edges of hex nuts
I figure that if some paints come with steel bb's inside the bottle from the factory, then using BB's and shaking must be okay with them (Alclad's primer and Aqua clear both have MANY steel beads in each bottle, so many that they make a swooshing noise when shaken)
but, I also spend the money to replace the caps on my new Aeromaster and Polly Scale paints,,,,,,so, I definitely didn't take the cheapest route
I have a "non-hobby" beauty section product that I use for a battery operated paint stirrer, in addition to the Tamiya stir stick, and I shake my paints to mix all the ones I will use that day, battery stir them as I open them for actual use, and then stick stir them as needed while painting
side note,,,,,,a lot of the McD's don't have the stir sticks anymore, they use a skinny straw product to save on operating costs (per 5 McD managers in the Nashville area when I asked them),,,,,you can get a bagful of cocktail stir sticks made of plastic, one type has a bead sized ball on the stirring end
Rex