[Jack21]

Here's the problem I've run into with HPC type coatings. First off, the HPC is nearly manditory on your headers and "J" pipes leading to the muffler, inside, and outside.

Once you get to the mufflers, it's not the coating, but the polishing that causes problems inside the muffler. The polishing is done in a vibrating water/detergent bath filled with polishing media/beads. This water bath matts the fiberglass, and rusts any steel wool packing. Either seal the muffler innards during polishing, use stainless steel wool for packing, or don't coat the mufflers. Use VHT paint on the muffler exterior instead.