Having the prototype available before the first use of the function allows the compiler to check that the correct number and type of arguments are used in the function call and that the returned value, if any, is being used reasonably. The function definition itself can act as an implicit function declaration.
While shellac is not usually the finish paint choice, it is often thought of as the best paint primer available. It has properties that oil and water based paints can't match, and provides a more dependable seal than most other primers are able to provide.
I probably wouldn't use nail polish, but I would use some shellac or whatever from Home Depot or Lowes. That stuff is made to not come off. and it is heck of a lot cheaper than nail polish. Shellac is about the least water resistant finish you could try.