I use a variety of methods to clean fretboards. This one is the best I know of for a dirty fret board. First take your strings off and use a guitar pick or your fingernail(covered with a cloth) to rub on the fret board. The pick will scrape some off....
I think some strings rust or corrode faster than other strings depending on quality, materials, packaging, & manufacturer. I'm guessing it may also run into the untouched by human hands thing. I don't know if all manufacturers are careful about that when packaging strings. They obviously should be!
If you have a new set of strings unopened & sealed in plastic from a good manufacturer and stored in a decent environment, they should last a very long time. If the plastic package they come in isn't airtight, put them in one that is.


