dk nkf - Nordisk Konservatorforbund Danmark

c) Indoors at normal room temperatures
d) Or, seldom, in dry, special climate-controlled
areas.
Thin or thicker non-polar coatings on iron will at
best form unbroken film coatings that act as a shield
for some time and prevent the polar water molecules
from settling. The protectiveness of a coating depends
therefore on how completely it coats and penetrates
to give water-displacing properties. It should also
have good adhesion and withstand penetration over
time. The film adhesion and stability is, of course,
dependent on the preparation quality of the iron
beneath.
The ability of various clear coats to cover uneven
surfaces is described by several authors. Very
uneven metal surfaces and edges are difficult to
protect evenly with some types of coating [3].
Layers can turn out to be very thin on edges (more
and less sharp ones) as is mentioned about the
scythe in figure 1. For protecting outdoor iron it has
therefore sometimes been necessary to even out and
blunt edges before coating.
The properties of new oil products, waxes, lacquers
and paint that industrial scientists wish to develop
for coating iron are much the same as those the
conservator seeks. The scientists mostly develop
coatings for clean metal surfaces. Zinc oxide
compounds are often used as electrochemical
protection in paint primers while lead oxides now
are being abandoned. Furthermore inhibitors now
seem to be an important part of primers, clear and
semi-transparent coats. They can, at best, delay
the anodic and/or cathodic reactions of corrosion.
Coatings are developed for either long-time
protection or for short-term, such as for overseas
transport of machines and weapons. For our use, we
would also like to have products that are protective
on top of corrosion layers. Unfortunately, at least
some polymer films are sensitive to some metallic
corrosion products causing weaknesses and then
breakdown.
Another consideration in choosing coatings
involves removing them later. Some tend, over
time, to crosslink and discolour. Partial mechanical
removal with scalpels, as paint conservators do,
will often be considered too time-consuming and
expensive for iron objects. Immersion in a tank for
electrolysis or else steam and heat treatments are
other possibilities. These methods are without health
hazards for staff, but will take away, for example,
preserved older interesting paint remains and harm
attached wooden parts. Therefore not all objects are
suited for these treatments. Removal with solvents
can become a safety risk when dealing with large
surfaces, although newer paint remover products
should be less harmful. In removing the conservator
would have to consider saving parts of the original
paint etc. for later documentation or research by
taking samples.
A coating’s protection of iron against vapour
penetration can be judged just by immersing a
test piece in water. Although accelerated, this test
is reasonably reliable for comparing products.
Outdoor exposure tests on panels in sheltered or
un-sheltered locations will, on the other hand, give
the conservator an impression of protection time. A
general rule says the thicker the film, the better the
protection. This, of course, also has limits because
a coat’s expansion and contraction at changing
temperatures is greater, than that of the metal to
which it is applied. Coatings containing pigments
produce thicker layers than clear ones.
The tendency of coating to yellow and crosslink has
to be tested by exposure to light. Modern standard
tests combine several factors.
Contamination such as dust, dirt and microbial growth
(lichen) on coatings will keep the humidity high and
thereby accelerate failure as illustrated in figure 2.
To prevent time-consuming frequent repair and
renewal of coats on iron, it is practical to have some
guidelines. Following are mine in climate conditions
a-d (as noted above) until news from ongoing tests
is ready:
a) Unsheltered outdoors iron should be completely
cleaned and coated with several layers of paint.
One picks a paint system for outdoor iron with
correctly applied primer and subsequent coats.
The recommended light sandblasting of the metal
surface beforehand gives the best result. Sharp object
edges, as mentioned, must sometimes be rounded. A
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