Can't believe nobody mentioned having it professionally cleaned.

Any machine shop in your area can either hot-tank it, or glass bead it. To hot-tank, they basically submerge the head/parts in a tank of nasty caustic shit that eats away ANYTHING. Glass beading involves blasting it with high-velocity bits of glass or something, almost like sandblasting. Works great, and it'll get anything off.

IMO it's not worth trying to clean your own engine parts. A tanking/beading costs all of 30 bucks, takes no effort on your part, and gets it about 10x cleaner than ANYTHING you could ever hope to do on your own.

My local machine shop charged me 40 bux to tank AND bead a head I brought in. Here's before and after pix .. you be the judge.

Try doing that on your own.