fishfinder problems....

[fowl intent]

I bought a boat a few months ago which is outfitted with a Zircom fishfinder, supposedly a high tech (ie. expensive unit). Zircom was bought out recently by another major manufacturer, I believe Hummingbird. Anyway, the unit seems to be working at times, then for no reason will begin to register false depth readings. (600+ feet in 15 feet of water). Even when it seems to be working, I have never marked any bottom structure or fish, even when trolling over marked wrecks and artificial reefs. The transom mounted transducer was painted with antifouling paint when the hull was painted several years ago, and I assume that does'nt help the situation. I figure I had better take care of that problem before I try anything else. Any suggestions on how to clean the paint off the transducer without damaging it? Can I scrape it or use fine sandpaper? Paint thinner? Any other suggestions?