[Slopel]

Hello all, it seems i have run into a problem. My 65 germanlooker is entered in a car show coming up here the first weekend in June. It has some ways to go but while I was looking er over today i noticed the inside of one of my 2001 boxter alloys seems to be tarnished or kinda rusty looking?....before i put them on, one had to sit outside for lack of room in the garage and i thikn it rained and water sat in it. How should i go about cleaning this off?
thanks, -Brandon_