I'm not a fan of painting steel targets as I feel part of the challenge is making the rusty steel out against a field background.
But I have a few plates I like to check zero on, very difficult when they're covered in lead spatter, so I paint those

I bought a carton of spray cans a few years back, but I find them more annoying than helpful.
I think the ideal is probably a gallon can of left over white house paint and a four-inch brush, but that's messy and difficult to either leave in the field or carry out and back all the time. And opening it so often is not conducive to preventing it from setting hard in the can eventually, not to mention the brushes needing cleaning or replacing. Wooden brushes are only a buck apiece though which is cheap enough not to bother trying to clean them.
250gm spray cans are about $4 apiece, but can often be found on special in cartons of a dozen cans.
A 4-litre can of house paint is about $35, about half the price of spray cans. But I've found it also covers much better than sprays.
So, what do you use and why?
What do clubs use to paint their steel targets all the time?