To get them to where I want them, all rusty and grungy looking, I began with a dip in a bath of muratic acid, the kind used for swimming pools. It's diluted quite a bit and works good for this process although a respirator and doing this outside is a unpleasant to breathe.
I have always liked the look of painted antiques and how the paint seems worn (battered in some cases) and all the patina that naturally comes with age.
Some of the patina is difficult to emulate, while other aspects are fairly easy.
Dump the screws into the water, let them set for a few minutes then simply pour out the water and place the screws on a shop rag and let them air dry.
It usually takes 24 hours for them to be ready to use.
I have been doing the rusting process myself for several years and it's relatively easy and much cheaper, while not entirely safe if you don't take some precautions.
First you have to get all steel nails, screws, hinges or whatever you plan to rust.Antique, old, wood, wooden, primitive, screw style, table vise, furniture making tool, possibly a harness makers vise.This item shows overall wear including scratches, minor surface rust found to the heads of the nails, some splitting found to the interior piece of wood, small chips, there is a split found to the body of the long piece of wood which does not affect function, and dirtiness found.The handmade nails of the period derived much of their holding power from the ability to drive the nail through two surfaces and bend it over on the backside, i.e. But that solution would not work for securing the top on a chest of drawers or table top without either driving a nail through the top from above or clinching it on the top to hold it fast.The same problem arose while trying to affix a lock to the backside of a drawer.One such way to help make a new piece have the aged and more authentic look is to use rusty hardware (hinges, screws, square nails, etc).