Exterior air conditioning pipe insulation
Gloss white paint reflects most heat. 
a clean slick surface that does not hold dirt is best. Silver/aluminum reflects more BUT white re-emits most of the radiation it absorbs and silver does not . Acrylic white is not so good;great resistance to UV but it chalks more so the surface becomes dull/dirty.  I read this 40 yr ago in research for oil storage tankage, no hope of finding the reference. But when I see painted oil tanks today ,they are white so apparently the physics has not changed.