I found this older 357 Western Derringer in an old building that was going to be burned down by our volunteer fire department. It was completely rusted and had no grips. I soaked it in motor oil for about 2 weeks and then slowly started taking it apart. Had to heat every screw to get them out. Luckily barrel wasn't pitted. Made the grips out of cow horn. Wanting to reblue or do the darker brown two tone blueing you often see on some muzzle loaders. This little gun isn't very pretty but it's a prom queen from what it was when I found it. I've shot it several times. It's quite loud and will dang sure jump up in your hand some. I've shot 357 ammo through it but now just light 38 special wad cutters. I keep it in a truck where it's always about 6 inches from my left hand if I was ever in the situation to use it. I probably won't hit anyone that's trying to do me harm but I guarantee the muzzle blast will scare the hell out of them.
Is it an old chrome derringer or was it a blued model?
Birchwood Casey makes a great line of products to bring that gun back to its prior glory unless it's the chrome model. I recommend the hot blue method if the original finish is blued.
If it's chrome, your best bet is to try and polish the original finish with something like jewelers rouge on a cloth wheel thats ran with a bench grinder.
If the grips are wood, you can hit them with some 400 grit sandpaper to smooth them up and remove the old finish. Then move to 600 grit to get them really smooth and ready for a finish.