Strip the old finish, clean it off, then apply new finish.
The specifics depend on what the original finish was and what the new finish will be. Finishes can be very different. Some, like bluing, have whole books written about them.
One thing that all finishes have in common is the fact that surface preparation is the most important part of the process.
Ultimately, if the gun is valuable, take it to a professional. Metal finishing is part science, part art.
There are many types of finish. Your local library should have books on furniture/woodworking, etc..
Try gun shops, gun shows, want ads, e-gunparts.com
Home stores such as the Home Depot have specialists that can help you refinish your kitchen. They can give you tips and advice on how to refinish the kitchen within your wants.
Diminish.
I need to someone to refinish my hardwood floors. What is the average price of doing so?
One can find a guide to refinish hardwood floors online at various websites. One can find a guide to refinish hardwood floors online at Redbeacon and This Old House.
You can always use a derusting product, but the finish can be damaged. You may need to repaint and refinish.
There's a page on DIY Network that explains how to do this: http://www.diynetwork.com/how-to/how-to-refinish-a-bathtub/index.html
Organizations such as Pintrest and Hgtv refinish aging furniture. The prices of these services is very reasonable and the service quality is excellent.
You need a professional sander in order to refinish wood floors. It's easy to do if you have the right equipment.
Refinish cherry wood cabinets is a great home improvement project. This website, www.faceyourkitchen.com, has step-by-step tips on how to refinish cabinets and also recommends many products specific to the different wood types.
Can't be answered without knowing who made it and the serial number. don't refinish it.