Hi Steve,
In your first attempt, I think you did a job more appropriate for painting a fender. However, as you have indicated, the radiator does expand and contract so the paint needs something more to grab and your 1000 grit sanding made the surface to smooth to provide sufficient grip. I agree with Steveg as the radiator gets hot but within the limits of standard Rustoleum or Krylon spray paints (I used a satin black Rustoleum last painted around 15 years ago). I roughed up the tanks with a medium grit paper for the primer to grab. The primer filled in any roughness and provided a good adhesive surface for the final paint to grip.
Considering the red fan guard originally had brush marks in the paint from the factory, I would suggest you not overdo the finish on the tank.
All the best,
Ray(64BJ8P1)
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