If it leaks through the thru-hull fitting, then go to your local marine supply store (ex. West Marine) and get a tube of SIKAFLEX. Take the through hull apart (if its rivets, drill them out) then clean the surfaces real well. You dont have to worry about roughing up the surfaces. Mask around the exhaust hole, lay a nice bead on the thu-hull fitting and press it in, then squish it in tight. Get a paper towel with mineral spirits and wipe off the excess sikaflex. Then peel off the tape, and voila, perfect seal that will never leak, and will look factory. Remember to use Stainless rivets when re-installing the fitting.