Regular diluted car shampoo in spray bottle between wash. Spray on inside/outside tip, wipe with plain water damp cloth and follow with Costco microfiber cloth. Same method for my C6 Corvette's after market Catback Corsa Sport's exhaust tips.
Civic's 27Won front pipe back exhaust, from muffler to exhaust tips and Corvette's Corsa's exhaust tips were coated with Kamikaze ceramic, high temperature Wheel Coat two years ago. Hold up really well after two years, sure make life simple.