I've recently acquired a couple of 60s era Omegas. I've read advice to have the mechanism serviced every 5 yrs and to have a reputable jeweler clean the case twice a year. Specifically, how should the case be cleaned? I want to be sure to be clear in what I ask to have done. I'm sure neither is waterproof. One is 18k, the other is 14k/SS. Thanks.