Knock off the heavy, and reseason. With use, the rust will go away. Usually you won't have any problem if you use it.
On a water pan, that's a little different. You may need more oomph than a drill motor with wire brush. But if you don't want to buy or borrow an angle grinder that spins at 10,000+ rpm and use a wire cup brush on it (which removes loose paint and rust very effectively) and you don't have a friend with a bead blaster, here are some options:
1. get a big tub from the store and use your small battery charger to do electrolytic derusting (you can google this up, I'm not going to type it all in here)
2. same big tub, dump in two gallons of vinegar and a pound of salt, stir vigorously to dissolve as much as possible, put your water bowl in, add water if needed to cover, let sit 24 hours or at least overnight. The solution works a lot better if it's warm, so an incandescent light bulb hung down into the tub with plywood over the top keeps away bugs and will warm the bath a little. After doing this, you will need to paint or at least oil the water pan quickly, as chemically clean steel will rust nearly instantly in contact with any water at all.
seattlepitboss
