What does engine oil mixed with coolant fluid look like?c

I just retired this past summer, and I decided to take up a car restoration project. I’ve got the engine stripped down, but the cylinder head looks filthy. I don’t have a professional blaster machine, so could you explain how to clean a cylinder head without one? Is it doable?

I like to “top off” my gas tank, and sometimes I fill it up a little too much by mistake. It’s even spilled out onto the side of the car once or twice. Aside from being bad for the paint, is this a serious problem? Is it bad to overfill your gas tank?

I live in an area that's prone to occasional acid rain and it leaves a residue on my car windows. Obviously this presents a safety issue, and I don't want to keep driving with this residue if there's a chance it might get me into an accident. How can I remove this residue?