Tim, only water-based clears will work, anything else will ruin the chrome. Createx and Vallejo seem to work fine, but I've only brushed them on so far, which really doesn't work well. But they don't effect the chrome at all, so get one of them, they can be airbrushed easily. I'm not sure how well they polish up though, I've never used them before!
Thanks for the assist Ed!