You need a good quality "electronics" cleaner/degreaser spray solvent made special for cleaning relays and switch banks.
One such chemical I highly recommend is Chemtronics "Electro-Wash NR" (manufacturer # ES1614). Evaporates very fast. Available from MCM Electronics as their stock # 20-4040. Be advised that most of these kinds of products cannot be shipped by aircraft (ground shipping only). Some state regulations only allow sales of these products to professionals. If your state resricts sales, you can try Radio Shack # 64-4327 Electronic cleaning spray. It's just 'ok' and will probably work for you.