Is there any product I can use on old silver plated jewelry that the plating is worn off? Like a paint or?
Having your jewelry professionally replated is the best way to repair damaged or worn plating. Visit your preferred jewelry store to see what the cost would be and then consider getting a second quote before making a final decision.
For a temporary solution, you can try the following to help disguise the affected area:
Brush on a layer of Mona Lisa™ Metal Leaf™ top coat sealant, apply a piece of silver leaf sheet then seal the leaf with Mona Lisa Metal Leaf for protection of the leaf surface and resistance to tarnish. Obtain a thicker metal texture by applying two or more layers of metal leaf. Another affordable solution is to use Krylon brand leafing pens to temporarily camouflage the damaged area. They are available in five metallic shades including silver, and are advertised to adhere to wood, metal, plastic, glass and more.
Another option is to apply a Swellegant!™ metal coating. This line created by Christi Friesen allows almost any material to be coated with metal using a brush-on technique.
Either option will leave you with metallic color-match issues to deal with and you may have to cover more than just the damaged area for color blending. Keep in mind that a camouflaged/ modified piece of vintage or costume jewelry may lose monetary value.