rogpatt,
In my ongoing attempt to say anything about everything, I will say this. The generic rattle can paint is good to about 120 degrees.
I would imagine most parts of the engine get that warm, as the coolant temp usually runs more than this.
If one were going to take the time and energy to paint the engine, and with so many colors available in 'engine paint', why not use it.
I think most engine paints go to 300 degrees, and would be fade free up to this temperature.
That's my take on the subject.
Stuart
