"Application Note: While Chassis Saver has incredible rust and corrosion resistance and can be used for all underbody applications without top coating, it is sensitive to UV light (sun) and its initial appearance will change over time. Its “RUST STOPPING” properties and corrosion resistance will never degrade but its color will shift from black to charcoal gray. Silver-aluminum remains very close to it’s original appearance. If aesthetics are important, Chassis Saver can and should be top coated with a quality industrial or automotive finish. Any opaque finish is effective. Clear coating is not recommended as protection. Suitable topcoats include any MAGNET Finishing System including 4800 Series Synthetic Urethane Enamels, MAGNACRYL™ 9000 Series Acrylic Enamels or MAGNATRON™ 5000 Series Acrylic Polyurethane Enamels. Call customer service or ask your local distributor for more information"
It looks like you can top coat this product as you would with POR 15. This was taken from the Magnet Paints website, perhaps the technical rep you spoke to was mistaken?
http://www.magnetpaints.com/underbody.asp