I keep a two part epoxy (as used with fiberglass) for problems like this. The epoxy can be thinned down with denatured alcohol if necessary and in this case would probably result in a coating similar to what arbutus is describing. Don't expect to remove the plug afterward though 
Shelf life is very long (years). Tons of uses as either a glue or coating. You might find it at a auto store -- just remember it's epoxy and not polyester resin which is generally just a huge PITA.