It's aluminum. Take it to a place and have it anodized black. It will be stronger, and there won't be an extra "layer" of paint to insulate the fins.

 

there is a product from swain thermo coatings that paints your radiator black and actually increases its efficiency. Good luck finding it though

 

You beat me to it... Anodizing won't change the thermodynamics...

 

I read a magazine article years ago where they painted several identical air cooled VW motors different colors to see which color would run cooler. They tried black, silver, red, white and non painted. The black motor ran cooler than all other colors. It even ran cooler than the non-painted one. It went on to say the motorcycle engineers have known this for years, thus the black motors.

 

it's been proven that painted intercooler works just as well as non-painted one.

i am sure there's a drop in air flow due to restriction but it's been tested for intercooler that there's no significant change

 

I think they sell radiator paint. You should check it out.