Rust converters are primarily designed to be used on steel and iron surfaces that don’t have any coatings on top. These coatings (which are designed to fight rust themselves) interfere with the chemical process that the converter is designed to create. That means stainless steel isn’t usually compatible with steel converters - nor is galvanized steel. Oil-based converters may be more useful here, but we don’t recommend it unless instructions specifically mention stainless steel. On a similar note, not all corrosion is rust. Materials like aluminum, copper and silver can all corrode. But rust converters won’t help with these materials at all.