I run marketing for a company that makes rodent and pest exclusion products. In the course of our product development we've consulted with multiple rodentologists and done a bunch of our own testing and as @carreatime discovered, repellants are for the most part ineffective. Some may have a short term effect as rats are generally neophobic but once they get used to it it's game on again. The only sure things are trapping them before they get in or putting up a physical barrier that they can't chew through, climb over or dig under. This is what our stuff does but we are primarily focused on keeping them out of structures (houses, apartments, warehouses, etc.).We've had rats dining in our engine bays of our Jag and Lexus. Gets expensive. Insurance will help. We've tried electronics, mothballs, mint spray. Nothing works. I find rat poop on all the devices and treated areas. Trapping them before they get in seems the most effective.