Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

I'd just de-grease and spray the boiler with some heat resistant paint, and leave the bottom part sitting within the firebox unpainted.
Downside to just spraypainting as is, would be that the paint obviously won't stick extremely well, to such a smooth surface.
But if handled responsibly, I think it will suffice for normal use.
Upside would be, that the paint can be very easily removed again with paint thinner or similar solvents, and the boiler returned to original condition.