Bite the bullet and get some ceramic coating,
Its not that expensive really, mine is getting, done at the moment
I'll get change from a grand coating the pistons, combustion face (including the base of the valves) the pre combustion chambers inside and out, inside the exhaust runners, the exhaust manifold inside and out, the turbine housing inside and out and the dump pipe inside and out.

Thats a lot of ceramic for less than a grand.