We only apply our ceramic coatings to the external surfaces. This is because the conditions inside an exhaust manifold are extreme with gas at temperatures in excess of 1,000oC, pulsing gas flow of more than 20m/sec, large pressure and temperature variations, significant vibration and thermal stress. It is impossible to prepare the internal surfaces of a pipe or manifold for these conditions. With such arduous operating conditions we fear that these products can easily become detached, causing serious damage to downstream components such as turbochargers and catalysts. Furthermore, these products narrow the flow channels and can restrict flow, giving rise to reduced engine performance. Our recommendation is that you limit coating to just the external surfaces. You might consider polishing the internals to avoid any rough edges or corners, but our technical experts really do not advise putting any form of coating on the inside of an exhaust manifold or exhaust system.