Derek said:

> I have seen aqeous dispersion paints blistering off hives in Portugal.
> Whilst they allow the escape of water vapour, their wet adhesion to
softwood
> is I think questionable.

As I mentioned in my original post, water based paints require "keying"
(sanding) of the surface to adhere well (a step many omit).  The only
peeling I have ever experienced in 14 years of professional use was when I
did not sand/scuff the material first.  I also specified 100% acrylic latex.
Not all water based paints are of equivalent quality.  Many contain vinyl
compounds which degrade.  If one is really opposed to sanding, oil primer is
an option, but that renders the paint film impermeable, and cancels one of
the benefits of acrylic latex.

>The Scandinavians have long experience in painting
>their houses constructed of softwood. They use what the Germans call
>Holzlasur - based on long-oil alkyd resins

Yes, I forgot that we "inexperienced" Americans have only been painting our
houses for around 400 years :)

Although long oil resins (slower drying) last longer outdoors than short oil
(faster drying), alkyd (oil base) paints all share the same vulnerabilities:

1)  Oil base paints are severely susceptible to UV degradation.  This is why
oil base paint films "chalk" and fade.  100% acrylic latex resins are
*immune* to UV.

2)  Alkyd paint films continue to cure indefinitely, getting harder and more
inflexible as time passes.  Eventually the paint film becomes brittle, and
can't absorb the radical expansion and contraction of wood.

3)  Alkyd paint films are more susceptible to mildew, thus the need for
higher levels of protection (biocides) than acrylic latex paints.

4)  With the newer VOC (volatile organic compound) limitations, oil based
paint quality has suffered relative to it's water based counterparts.

I have used the finest paints of both types available anywhere (including
those containing lead, which further enhances "weatherability") in Zone 3-4
(-40F winters) in the context of road-side wooden signs, which suffer the
worst summer/winter abuse imaginable.  I almost never use oil based paints
today.  I can say conclusively that nothing compares to the new generation
100% acrylic latex paints.  If you ask any paint tech. worth their salt,
they'll pretty much all tell you the same thing -  on unpainted wood, 100%
acrylic latex will far outlast alkyd/oil base paints.  High performance
water based acrylics are even used on ferrous metals today.  The only
advantage that oil paints offer is higher resistance to abrasion.


Regards,

Todd.

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