Your mirror may have scratches in the glass or in the silvering on the back; either way, if you consider the mirror valuable enough to repair, you'll have your work cut out for you. It's possible to remove surface scratches in glass by buffing them out, but this technique won't work if the scratches are deep. When it comes to repairing scratches in the silvering, you can't simply spray new silvering over them because the scratches will still be visible. The only method that really works is to strip the old silver and resilver the mirror.
Removing Scratches by Polishing
Step 1 Remove the mirror from the frame.
Be careful! Whether or not the mirror is an antique, it has probably been in the frame for many years and may be stuck. Work the frame carefully away from the glass to prevent damage to the glass. Lay the glass face up on a clean, flat surface protected by cardboard or felt.
Step 2 Clean the glass.
It's important to remove any small particles that could be ground into the glass during the polishing procedure. Spray the glass with window cleaner or a vinegar/water solution and squeegee the glass clean.
Step 3 Polish the scratch.
Put a fresh lambswool pad on an electric buffer and apply a small amount of cerium oxide, otherwise known as jeweler's rouge. Operating the buffer at a moderate speed -- 2,000 to 2,500 revolutions per minute -- press the pad lightly on the glass and run it over the scratch to buff it out. If the scratch is near an edge or bevel, you may prefer to buff by hand. In either case, take frequent rests to allow the glass to cool down -- overheating the glass may crack it.
Step 4 Clean the glass.
Remove polishing compound with a solution consisting of a tablespoon of vinegar in a cup of warm water. Spray the solution on the glass and wipe it off with a clean cloth or paper towel.
Step 5 Try again.
Repeat this procedure up to three times to remove moderately deep scratches. If you can't get a scratch out after three times, get a professional evaluation of the feasibility of removing it.
Resilvering a Mirror
Step 1 Set up the mirror.
Lay the mirror face down on a clean, flat surface protected by cardboard or felt. No part of the glass should be hanging over the edge of the surface.
Step 2 Strip the backing paint.
Use a product designed for removing paint from mirrors. Spread it on the back of the mirror with a plastic paint scraper, paintbrush or paint roller, let it sit for eight to 12 hours and scrape it off with a plastic paint scraper. If you have a hard time scraping paint, the stripper didn't sit long enough -- apply more and wait for a longer period before scraping it off.
Step 3 Dissolve the silver and copper.
Wipe the glass with cotton swabs and mirror remover -- a solvent that dissolves the reflective metals that create the mirror effect -- until the glass is clean and completely transparent.
Step 4 Clean the glass.
The glass must be absolutely clean for resilvering to be successful. Spray glass cleaner on a rag, and use the rag to wipe the glass; don't spray anything directly on the glass -- water interferes with the resilvering process.