Is overspray on cylinder head ok?

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Correct! I apply Glyptal on almost every motor I build. If used correctly, it has its benefits.

I would personally pull OP's motor apart. Details matter. If a builder is willing to skimp on the parts you can see on the outside, just imagine what you can't see on the inside.
You took the words right out of my mouth. If the builder cut corners on something that can be seen so easily, where else did he cut corners?????