Yes, alcohol is good, but if the contacts have been arcing, there will be metal deposited from one contact to another, you can use a very fine file, or even a nail board to flatten and polish the contacts. they should be left clean, and slightly domed. A thin coat of vaseline helps to damp arcing and allows them to slide over each other to find their closest contact point. Many will say you can't do this, I was taught it as an apprentice electrical engineer, and have done exactly this for over 40 years. It stops arcing, and noisy starters, and is a one time cure!