While a coating of paint can prevent a good electrical connection I would never trust it to provide adequate electrical insulation. But clearly we need a more complete description of what the intention is. OR did you mean thermal insulation??
Most old tube amplifiers didn't even use anything just bare metal. But if you really think it's needed I'd use the DIY bed liner. It is much thicker than regular paint and harder to scratch through. And should insulate better.
Have the chassis powder coated. The powder is sprayed on and baked at a high temperature. I think it's about 400F or so.
For touch-up you can get regular paint in the same color. Usually a RAL color.
Certainly the Rustoleum product gets my vote as well,, even the brush-on enamel paint. But I still say that it is a poor choice to depend on a coating of paint for electrical insulation. It might work for a while but it CAN NOT be trusted.