How do I refurbish door knob plates?

If they are brass then ketchup does an excellent job

Try SOS pads

Automotive paint , comes in great colors and you can keep the extra and use for any touchup.

no sos pads on brass That will scratch it. Use a product called Brasso. Wipe it on, let it dry, buff it off with a soft cloth.

The long term fix for your brass is to have it re-electroplated. You would need to remove all the hardware you want refinished and ship it to an electroplating business. Do a search for that.

Otherwise, you have to choose a metallic paint then put a clear coat over that. No, it won't last forever, but it should last for many years. But you are looking for an easy, inexpensive fix. You should be able to find what you need at a building supply, hardware, or hobby store. Below are a couple of sites with good info.

https://www.infarrantlycreative.net/spray-painting-door-knobs/

https://www.younghouselove.com/you-down-with-orb-yeah-you-know-me/

I've used Rub n' Buff on different projects for years. I rub it on, let it dry and, polish it off with Kleenex or similar (it is not shiny). It is an oxide mixed with oil and, works like a paste. It comes in a variety of colors Gold, Antique Gold, Copper, Spanish Copper (very dark) - may be right for your project, Ebony, even found a blue that looks like Verde Gre(?) on our plastic, copper color dimpled mirror frame. I used Ebony to change a brass fire screen into black. I've purchased it at Hobby L, Michael's, and Amazon, as well as, other craft stores, you may have to call or check on the I-net. The size of the tube is tiny but, it really spreads a long way and is pretty tough. I use Goof Off to remove it. I think the tubes are around $10.00 or less but, please do check. Amazon wanted $37.00 per tube (no joke) but, I found it on line for $7.00 + $6.00 shipping. Experiment with one or two coats. It's good for stenciling and other projects.

Use metal spray paint but before that remove the door knob and clean it properly. Let it soak overnight in water with soap. That will do the job. Goodluck!!

Take them to a powder coating company that has a variety of colors and finishes.

Find a reputable plating company, like an automobile bumper exchange, as many do brass plating as well as the chrome plating. If not nearby, you may make arrangements to mail them to the plating company, as the knobs and plates are light enough that they may be economically mailed. Yes, I realize that this is NOT exactly DIY, but you will get newly re-plated and shiny new brass that will last for another few generations into the future.

Hi Mary, maybe you could do a peel and stick on them, make it any color you want but won't stand out too much,

https://www.amazon.ca/Adhesive-Peel-Stick-Removable-Wallpaper-15-6inch/dp/B07DVZSMWM/ref=sr_1_2?dchild=1&keywords=peel+and+stick+tarnished+brass&qid=1612808318&s=kitchen&sr=1-2

https://www.amazon.ca/Aspect-Backsplash-Brushed-Kitchen-Bathrooms/dp/B00GRADZGS/ref=sr_1_15?dchild=1&keywords=peel+and+stick+tarnished+brass&qid=1612808364&s=kitchen&sr=1-15