Quoting from Congressional Cemetery regarding tombstone repairs . . . Vinyl, latex, silicone, "Crazy Glue", "Gorilla Glue", epoxy, tar, caulk, and sealants are prohibited.
Quoting from findagrave.com . . .
How do I clean a headstone or grave marker?
Unless you are related to the interred on the headstone in question, DO NOT do anything to the headstone.
Gravestones should never be cleaned with anything but water and a soft brush. . . .
Never apply bleach, ammonia, shaving cream, chalk, flour, baking soda, cornstarch, firm pressure or use anything abrasive. . . .