I put rec.woodworking back in the header, figgered a few would find this
useful.
--
EA


"nestork" wrote in message
...

The reason they recommend leaving pressure treated wood to dry for a
while before painting it is because the pressure treatment process
involves injecting water soluble chemicals into the wood under pressure.
And, it takes time for the water forced into the wood to evaporate out
again. Also, weather conditions vary and some paints (like
top-of-the-line) exterior latex paints will allow moisture to pass
through the paint film without lifting, whereas any kind of oil based
paint will be lifted off the substrate as the water inside it evporates.
So, there's no such thing as a "one drying period" fits all solution.
Better to get a wood moisture meter and wait until the wood dries to a
19 percent moisture content, which is the same as kiln dried lumber.
It should be paintable then.

'Existential Angst[_2_ Wrote:
;2935537']Is there an exterior polyurethane that can protect pigments
form UV? Or are there UV-impervious pigments?


Exterior wood stains and clear coats will use transparent metal oxides
for UV protection. These metal oxides are transparent and either
colourless (clear), yellow, orange or brown in colour under visible
light, but are opaque to UV light. So, they help add a warm "amber"
look to exterior coatings to be used over wood, but they block UV light
from penetrating through the coating to damage the substrate.

But to answer your question, there are pigments used in architectural
paints that are unaffected by UV light, and that would never fade. But,
that doesn't mean that the apparant colour your eye sees won't change.
In southern climates, the intense UV light will cause the paint binder
to deteriorate, causing the paint to "chaulk" and the result will be
that the paint will appear to fade in colour as the surface gets rougher
and rougher.
Similarily, interior latex paints will gradually darken as airborne dirt
particles accumulate on the paint's surface.

Here's a post I wrote up a long time ago when I was posting on a
different forum:

ROCKS ARE GOOD AT BEING OPAQUE, BUT THEY'RE EVEN BETTER AT BEING OLD.

By the time you finish reading this post, that statement will make much
more sense than it does right now, and it'll help you decide on which
colours you want to see in your exterior paint's tinting formula, and
which ones you don't.

There are different kinds of pigments used in architectural paints;
coloured pigments determine the colour the paint film dries to, and
extender pigments determine the gloss level the paint film dries to:

There are two kinds of coloured pigments:

A. Organic Pigments (produced from chemicals in a lab)
The pigments used to tint architectural paints to the correct colour can
be broken down into two categories; organic and inorganic, and each of
those categories can be broken down into natural and synthetic. While
artists often prefer natural pigments, the standard for architectural
paints is synthetic organic and synthetic inorganic pigments which can
be manufactured to precise colours repeatedly.

In general, it can be said that organic pigments are the "colourwheel"
colours like red, blue and yellow, and all the secondary colours that
can be made by mixing these three primary colours, like green, orange,
purple, magenta, etc.

On the one hand, these organic pigments don't have good hide, but they
disperse well in the paint so a small amount of colourant goes a long
way to both change the colour of the paint as well as hide the
underlying colour.

Also, these organic pigments aren't as stable, and tend to fade more
rapidly due to exposure to the Sun than inorganic pigments.

B. Inorganic pigments (synthetic equivalent of coloured rocks)
The inorganic pigments you'll find in a typical paint tinting machine a
1. White - which is titanium dioxide
2. Yellow Oxide - which is a mustard yellow in colour and is the
synthetic equivalent of "Raw Sienna", which is named after the Italian
village of Sienna where the rocks and soil are a mustard yellow colour.
3. Red Oxide - is reddish brown in colour, and it's the most common form
of iron oxide. The Planet "Mars" is reddish brown in colour because of
the iron oxide in the rocks, sand and dust on it's surface.
4. Brown Oxide is another iron oxide that's very close to chocolate
brown in colour.
5. Raw Umber is a very dark brown that can almost be mistaken for black,
and
6. Black - which is actually soot made by burning natural gas is special
furnaces with insufficient oxygen so that copious amounts of soot are
formed.

All of these "inorganic pigments" are the synthetic equivalent of the
pulverized rock dust that artists have been using to paint with for
millenia. Basically, they are synthetic rock dust that has all the same
properties as pulverizing a real rock into dust, and using that dust as
coloured pigments in a paint to give that paint colour and opacity.

Because rocks are pretty good at being opaque, paints that get their
colour from rock dust tend to hide better than paints made from organic
pigments like red, yellow and blue. Unfortunately, one of the problems
with inorganic pigments is that they tend to want to clump together, and
it's this clumping together of inorganic pigments that lowers their
effectiveness at changing the colour of paint when tinting, and hiding
an underlying colour. But, all things considered, inorganic pigments
(best though of as pulverized rocks) make for better hiding paint than
organic pigments.

Since synthetic red oxide pigment contains the same rust molecules that
you find on cars, the synthetic and natural versions of inorganic
pigments have identical properties. They hide equally well and are
damaged by the same kinds of things; like acids. So, when we talk about
synthetic paint pigments, it's not a stretch to talk about them as
pulverized natural rocks.

Now, ANYTHING that is 300 million years old HAS TO BE extremely
chemically stable or it would have decomposed by now. That chemical
stability manifests itself in the fact that rocks are also extremely
colour fast. If you turn over a rock that's been sitting in the Sun for
100 years, after cleaning and drying, you'll find the bottom is the same
colour as the top! Rocks don't fade due to exposure to UV light, and
neither does paint that gets it's colour from synthetic rock dust. So,
since the planet Mars is just about exactly the same colour as the rust
on my car, it's fair to say that Mars hasn't faded at all in the coupla
billion years it's been exposed to the Sun.

So, pulverized rocks make for a better hiding paint, but it's the
extreme chemical stability of rocks that make them immune to fading due
to exposure to the Sun. That is, the shear age of rocks is an
indication of their extreme chemical stability, and that chemical
stability manifests itself as resistance to fading from exposure to the
Sun when you pulverize a rock and use the powder to provide colour and
opacity in your paint.

WHITE paints deserve special mention. There are different kinds of white
pigments used in paints. Titanium dioxide is the most commonly used
white pigment, and it has the highest hide of all white pigments except
lead carbonate, (which they haven't used since 1974). However, the hide
you get from titanium dioxide will depend on not only how much titanium
dioxide is in the paint, but how well dispersed it is and how large
those titanium dioxide pigments are. If they're too large, or too small,
they won't "diffract" light well, which means that light won't bend much
when it passes near a titanium dioxide pigment particle. So, different
white paints can have different hiding ability just because of the
amount of titanium dioxide in the paint (and generally the more TiO2 in
the paint, the whiter the paint will be in colour). But, even paints
with equal amounts of TiO2 in them may have considerably different
hiding ability just because of the dispersion and size of the TiO2
particles in the paint. The problem with using TiO2 in exterior latex
paints is that it acts as a catalyst in the process by which exposure to
intense sunlight causes paint binder resins to "chaulk". So, a paint
that's white because it has white titanium dioxide pigments in it will
chaulk more outdoors than a paint that's white because it has white lead
carbonate pigments or white zinc oxide pigments in it.

Consequently, in exterior paints intended for sale in southerly
latitudes (like Texas, Florida and Southern California) it's most common
to use zinc oxide as the white pigment in white, off-white or pastel
colours than titanium dioxide.

Zinc oxide doesn't have as good hide as TiO2, but like copper, arsenic
and boron, zinc is a natural fungicides, so using ZnO as the white
pigments helps suppress the growth of mildew and mold on light coloured
exterior paints. And, of course, the degree of dispersion and particle
size are also important in paints that use ZnO as their white pigment.

Titanium dioxide is the second highest hiding pigment used in house
paints. The highest hiding is black, which is actually carbon soot.




--
nestork