painting an intercooler - Heat Transfer 101

[rshydo at ithacamechanical.com]

I apologize, but I'm about to dork out here.  Please tune out if technical
things bother you.

If it were me (engineer speaking) I would anodize.

As far as heat transfer is concerned, those that say any coating will hurt
thermal performance are sort of correct.  (only sort of).  If the parts
are new and shiny, they are right.  If the aluminum parts have hung out in
the front of a car for a while with water, road salt, fuels, acid rain etc
hitting them, the aluminum will start to corrode and form aluminum oxide. 
Al203 is a pretty darn good insulator.  Even better if it blooms in funny
shapes that hold air (an even better insulator) near the aluminum.  That
kind of corrosion will definitely affect the heat transfer (and electrical
conductivity if you care).  Technically, any coating would solve that
problem if it were to remain intact.  Paint can be a bit thick, possibly
adding a little thermal resistance.  But Rob, isn't anodizing aluminum
oxide too????  Yes.  BUT the anodizing process creates a very thin,
controlled layer of it .0007 to .002" thick preventing it from making
those big ugly corroding aluminum blooms.

So as far as conduction is concerned, there probably won't be too much of
a difference is concerned between good paint and anodizing.  However,
people often forget that the main vehicle for heat transfer in an
intercooler is actually CONVECTION!  For convection to be effective you
need air flow and surface area of conductive materials.  My principal
concern with paint involves obstruction of air flow.  If you put on too
much paint you can end up blocking air flow through the small channels
that make the intercooler work like an intercooler.  The paint is also put
onto the metal, as opposed to being grown part way into it like anodizing.
 That would make flying rocks, sand, salt, small birds etc more likely to
loosen the paint, have a chip of it fly off and clog one of those little
passages.  Paint is also softer meaning that if dirt were to hit it, it
could stick and start to clog off the passages.  As that sucker plugs up
it conducts less and less heat, increasing IAT's.

In the long run, the effectiveness of paint will depend on the type of
paint, the surface preparation, and the quality of the paint job.  I would
personally be worried that the outer parts of the intercooler fins would
get coated but the middle ones would remain uncoated and corrode.  The
corrosion will eventually work its way out and flake the paint off of the
rest of the intercooler (look at boat outboard motors and such for
examples).  To really effectively prepare aluminum for painting it is
either dunked to remove oxidation or coated with something that stops it. 
Well, at that point if you are dunking and coating it already, anodizing
is just putting it into the next tank over at the processing facility and
letting it soak for a while......  Most of the effort would be in cleaning
to paint and the anodizing already has that effort included.

sorry for the book

Rob