My car already has a different type of rustproofing applied on it, will Rust Check be as efficient in this case?

Yes. In cases where an alternate product such as wax or tar based treatment has been used prior to Rust Check, those products typically show signs of failure after time. Drying, cracking and eventual flaking generally occur allowing moisture to creep beneath the coating, causing rust. Rust Check if applied over these coating also creeps in behind and pushes out the moisture, thus attacking the spread of rust.