Re: reflecting on hood materials

["S. Hieber" <shieber at yahoo_com>]

Wayne Jones, correcting my misunderstanding said:

> Scott wrote:
> 
>  Aluminum flashing from Home Depot won't reflect better
> than good white paint but it will do about as well as Reynolds
> aluminum
> foil.
> 
> -
>
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> - --------
> 
> By aluminum flashing I mean aluminum flashing that is painted white
> in
> the factory. It's probably powder paint but I don't actually know
> what
> process they use. Even in direct sunlight it is still pretty good
> after
> 30 years or so. Check out the aluminum facia on an older house.
> Unpainted aluminum flashing would make a very poor reflector.

Yes for white paint, no for aluminum surface as a reflecting surface --
even before it is weathered, it's not as as refective as white paint.  
White painted aluminum is a dureable combination.
> 
> Any solvent based paint that cures by something evapourating from it
> will not be impervious to moisture. This includes latex, oil base or
> air
> cured polyurethane. 
Absolutely true -- shellace and lacquer are the worst, and urethanes
the best, but even those will pass moisture in and out of wood as the
humidity changes  -- that's why it's wise to coat both sides of the
wood, to redcue the tendency to warp from seasonal humidity changes.  
A hood over water is bound to see a lot of moisture. 

If your hood is over glass, and is well coated with solvent-based
urethane, it will probably last for years and years without problems.

> If you are using the paint as a difuse reflector
> then you really need something that will stay pristine or the
> effectiveness will be dramatically reduced. I have my doubts that
> white
> epoxy would contain enough pigment to be highly reflective. The
> pigment
> is trapped in a bed of clear epoxy and it seems to me light would be
> trapped in the epoxy coating between the pigment particles.

You want something to reflect and something to seal.  To get the same
out of one thing, paint is your best choice.  AH reflectors in a
"conversion finish" coated hood is your best choice if you want wood,
have lots of money, a spray booth with explosion-proof motor on the
ventilation system, etc.
> 
> Epoxy may appear to be expensive because of it's high cost for a
> given
> volume but it is not really. It just doesn't contain any solvents
> that
> will evaporate off anyway. You can do a large hood with about a quart
> of
> epoxy. Each coat of epoxy is like 10 coats of a solvent based paint
> but
> even if you were to put 30 coats of solvent based paint (which would
> take weeks to apply and would leak fumes forever) you still would not
> have a waterproof barrier. For protection epoxy is the material of
> choice. For reflectivity, I think powder painted steel or aluminum
> would
> be the right choice. It's what they use in fluorescent fixtures
> anyway
> and it's cheap. 

Well that's the key reason they use it, that, and it never shows a need
for dusting ;-)

Scott H.

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