We use isopropyl alcohol at work (rubbing alcohol). Don't go for anything fancy. Make sure when you clean it that none falls on your contacts since these pastes are conductive and will short things out for you. If it is stuck there and hard to remove, go to a hobby shop and get a small paint brush (1/4 inch wide) with stiff bristles. Soak it in alcohol then put a paper towel or soft cloth on top and use the brush to scrub the stuff away. The cloth will absorb the alcohol and residue and prevent scratching.