Because painted surfaces are subject to abrasion, soiling,
water damage, sunlight, and application of incompatible paints
they generally need to be repainted or at least reglazed appropriately
from time to time.
From the baseboards up to a level of about six feet off
the floor, wood trim is constantly subjected to wear from
being touched and inadvertently kicked, and from having
furniture pushed against it. Chair rails were in fact intended
to take the wear of having chairs pushed back against them
instead of against the more delicate plaster wall or expensive
wallpaper. Doors in particular, sometimes beautifully grained,
receive extensive handling. Baseboards get scraped by various
cleaning devices, and the lower rails of windows, as well
as window seats, take abuse.
The paint in all of these areas tends to become abraded.
Two things are important to bear in mind about areas of
- Samples taken to determine original paint colors and
layer sequences will not be accurate except at undamaged
- Also, dirt and oil or grease need to be removed before
applying any new paint because new paint will not adhere
to dirty, greasy surfaces.
Soiling is another problem of interior paint. Fireplaces
smoked; early coal-fired furnaces put out oily black soot;
gas lights and candles left dark smudges. Sometimes the
dirt got deposited on plaster walls or ceilings in a way
that makes the pattern of the lath behind the plaster quite
clear. Another source of dirt was polluted outside air,
from factories or other industries, infiltrating houses
and other nearby buildings. Until smokestacks became very
high, most air pollution was caused by nearby sources.
In paint investigation, dirt on the surface of paint layers;
as seen under the microscope, can be very useful in suggesting
the length of time a given paint layer remained exposed,
and in distinguishing a finish layer from a prime or undercoat
layer. This kind of soiling can happen on any painted surface
in a room, but may be slightly heavier in the recesses of
moldings and on upward-facing horizontal edges. Using dirt
as a sole measure, however, may be misleading if the surfaces
have been cleaned. The fracture or bonding between paint
layers is often used by professionals as a better means
of indicating time differences between layers as well as
indicating those layers that are part of a single decoration
Water, the usual source of deterioration for many kinds
of material, is also a prime cause of interior paint failure.
As a liquid, it can come from roof leaks, from faulty plumbing
or steam heating systems, or from firesuppression systems
that have misfired. As a vapor, it may come from such human
activities as breathing, showering, or cooking. Plaster
walls sealed with unpigmented hide glue are notably susceptible
to water damage because it forms a water-soluble layer between
the plaster and the paint. This can cause the paint to lose
adhesion when even small amounts of moisture come into contact
with the water-soluble sealer.
Finally, in historic interiors, especially where there
is heavy paint buildup, paint can weaken and fail due to
chemical or mechanical reasons. For example, the older linseed
oil is, the more brittle it is. It also darkens when it
is covered and gets no ultraviolet exposure.
In rooms where there is more sunlight on one area than
on others, the oil or even oil/alkyd paint will get discernibly
darker in the less exposed areas in as short a time as six
months. Painted over, the oil medium in older paints gets
quite yellow/brown, thus changing the color of the paint.
Prussian blue is one of the tinting pigments that is particularly
vulnerable to fading.
Understanding some basic differences in the strength of
various paints helps to explain certain paint problems.
Paints that dry to a stronger film are incompatible with
those which are weaker. Acrylic latex paints are stronger
than oil/alkyd paints. Oil or oil/alkyd paint is stronger
than waterbased paint such as calcimine. When a stronger
paint is applied over a weaker paint, it will tend to pull
off any weaker paint which may have begun to lose its bond
with its substrate.
Thus, on many ceilings of older buildings where oil/alkyd
paints have been applied over old calcimine, large strips
of paint may be peeling.
Oil or varnish glazes over older paints become brittle
with age, and can make removal of later paints rather easy.
Sometimes it is possible to take advantage of this characteristic
to reveal an earlier decorative treatment such as graining
or marbleizing. Getting under the edge of the glaze with
a scalpel blade can make the removal of later paints relatively
simple, and relatively harmless to the fancier paint treatment.
Sometimes, paints separate from each other simply due to
poor surface preparation in the past or the hardening of
the earlier surface paint.
Use of alkaline paint strippers can cause paint to lose
adhesion. When insufficiently neutralized, they leave salts
in wood which cause oil or oil/alkyd paints to fail to adhere
to the surface. If dirt or oily residues are not cleaned
from the surfaces to be painted, new paint will not remain