Re: What happens to the heat reflective value of aluminium when it is painted?

What an interesting question.  To answer your question, we need a little 
bit of background on heat transfer by radiation.  There are a number of 
informative sites on the internet that discuss heat transfer by 
radiation, such as the following:    http://physics.indiana.edu/~brabson/p510/selectivesurfaces.html
The amount of energy emitted by a surface is expressed by the Stefan-
Boltzmann equation: P/A = esT^4 where P is power, A is the area, e is the 
emissivity of the surface, s (sigma) is the Stefan-Boltzmann constant 
(5.670 × 10^-8 J s^-1 m^-2 K^-4, and T is the absolute temperature of the 
object that is emitting radiation.  All objects that are at a higher 
temperature than their surroundings emit heat (objects at lower 
temperature adsorb heat).  Radiation travels line of sight from one point 
to another, just like light.  And, just like light, radiation comes in 
various wavelengths.  

If you heat an object to a high temperature, like the tungsten filament 
in a light bulb you produce both visible and infra-red (heat) radiation.  
In fact, only about 2% of the energy that you put into a light bulb is 
converted into visible light.  If you put the same light bulb on a dimmer 
to decrease the amount of visible light, you also will notice that the 
light that is produced loses it's blue and green content, becoming yellow 
followed by red as the power is decreased.  If the dimmer is turned down 
further, you can still feel heat being produced by the lamp, but the 
amount of visible light is negligible.  So the total amount and the color 
(or wavelength) of radiation produced by a hot object depends upon it's 
temperature.  Higher temperatures produce shorter wavelengths, lower 
temperatures produce longer wavelengths.  Radiation that has a longer 
wavelength than visible light is referred to as infra-red radiation.

The amount of energy that an object emits is also a function of 
emissivity, which gets more toward your question.   A perfect emitter has 
an e value of 1, a perfect reflector has an e value of 0.  A polished 
aluminum surface has an e value as low as 0.04, whereas a heavily 
oxidized aluminum surface may have an emissivity of 0.2 or more (see  http://www.electro-optical.com/bb_rad/emissivity/matlemisivty.htm ).  

The emissivity and adsorbance of a surface are the same at the same 
wavelength.  So if a surface has an emissivity of 0.5, then it's 
adsorbance is also 0.5 at that wavelength.  But, materials have different 
emissivities at different wavelengths.  If you look at the following 
link, you can see the effect of different wavelengths of light on the 
amount of light reflected by different mirror surfaces:  http://www2.edmundoptics.com/techsup/MirrorCoating.pdf

Some materials that appear white in visible light (like typical white 
paint) have emissivities of 0.9 in the infra-red range.  In the visible 
range, white paint has an emissivity of 0.08 to 0.10.  Some materials are 
transparent to infra-red radiation, solar radiation, or both, but white 
paint isn't one.  However the following link:  http://www.umich.edu/~lowbrows/reflections/2001/tryan.5.html points out 
that having a low emissivity in the visible range and high emissivity in 
the infra-red range may be preferred to polished aluminum.  They point 
out that an aluminum surface, when exposed to a clear night sky only 
radiates a small amount of heat because of it's low emissivity, whereas a 
white painted surface tends to radiate much more heat at night, cooling 
down more than the aluminum surface.

They also point out that a preferred coating for your application may be 
an aluminized milar, where the milar has a high emissivity in the infra-
red for good radiation cooling at night, and the aluminum coating 
provides a low emissivity in the visible which will reduce the amount of 
solar radiation adsorbed during the day.  Plus, the mylar will help 
protect the aluminum coating from oxidizing.  So where do you find some 
aluminized mylar?  Use a Space blanket.  Get the cheap, silver/silver 
emergency blanket and just glue it onto the outside of your cooler (with 
the mylar or non-electrically conductive side on the outside.

Good luck with your cool project!