term "mildew" is a common term in the paint and coatings industry
and is used to describe an unsightly discoloration of a paint film.
Mildew is a general term for growth produced by mold fungi. This growth
can occur on a variety of surfaces both of organic and inorganic origins.
Surfaces can be inanimate, such as wood, vinyl, and aluminum; or living
materials such as plants. Mildew can also grow on superficial surfaces
such as dirt, grease, and other industrial pollutants, provided the
appropriate nutrients are present to facilitate such growth. Moisture
is the primary environmental component necessary for mildew growth,
followed in lesser degree by temperature. Consequently, tropical areas
that have high moisture (humidity) and high temperature profiles provide
the greatest geographic challenges to mildew growth prevention. Hot,
dry climates, as one would suspect, see much less mildew growth. Fungal
spores are present in air at counts of 100 to over 1000 per m3 of air,
depending on geographic location. Even freshly milled wood is immediately
exposed to mildew regardless of location. Approximately 100,000 species
of mildew exist, many of which affect the appearance and performance
of finishes. Mildew can be transported from one surface to another by
insects, animals, or air.
on Appearance and Performance
affects finishes in both appearance and performance. Mildew generally
appears in two forms, a spore type, which resembles caviar in appearance,
or a mycelium or filament type. Mildew generally appears as an unsightly
discoloration on a finish, thereby making the appearance unacceptable.
The performance of a finish may be comprised either by mildew growth
on the coating surface, or by the application of a coating to the mildewed
surface. The presence of mildew can have a detrimental effect on dirt
pickup, cracking, flaking, and adhesion properties of the finish. When
an appropriate finish is applied to a mildewed surface, the adhesion
of that finish to the substrate will be reduced to the physical interference
of the mildew. When an infected finish is applied to a substrate, the
above failures can occur even more rapidly.
are designed to have excellent adhesion to a variety of surfaces. Depending
on the paint manufacturer's intentions, a particular paint may be designed
to adhere to wood, masonry, vinyl, aluminum, and/or other substrates.
The presence of mildew or other foreign matters such as oil, grease,
dirt, tree pollens, and/or other substances on a substrate interferes
with the adhesion characteristics of the paint. It is in the homeowner's
best interest to ensure that the substrate to be painted is properly
cleaned and prepared prior to painting.
Adhesion to Paints
mildew has grown on a surface, a homeowner may think he can improve
the appearance by applying a coat of finish directly over the mildewed
surface. This approach is not a desirable cure. Instead, this new coat
of finish provides protection for the existing mildew, preventing its
removal, and can actually provide a nutrient source to facilitate new
mildew growth. Because of the infestation below the newly finished surface,
mildew will certainly appear again on the new surface. In addition to
the poor appearance of the mildew, a greater problem now exists with
the adhesion of the new finish to the old finish, a property commonly
referred to as intercoat adhesion. By not following good surface preparation
procedures, the homeowner has allowed the mildew to exist between the
old and new coats of finish. Intuitively and quantitatively, we know
that this mildew interferes with the new finish's ability to adhere
to the old finish. Early failures such as cracking, flaking, blistering,
can be expected. To insure the best intercoat adhesion, an appropriate
method of surface preparation must be followed. This is described below.
of Mildew on Wood
have discussed some of the harmful effects that mildew can have on wood
finishes. Mildew can also affect unfinished wood. In fact, it is often
more of a problem with unfinished wood. The colored residue that mildew
can deposit on a wood surface can cause severe discoloration; this discoloration
can be a variety of colors but is most often gray or black. However,
this mildew growth does not degrade the wood; mildew fungi are not capable
of using lignin, cellulose, or hemicellulose for food. Therefore, mildew
does not decrease the structural integrity of the wood. They can, however,
use the nonpolymeric materials in wood, such as the extractives and
natural oils for food. Wood species that are rich in natural extractives
may be more prone to mildew growth than wood species with lower extractive
content. Since mildew spores infect all surfaces, their growth can be
limited only by controlling moisture, temperature, or using paint film
the surface is infected with mildew, it can be removed using a mildew
cleaner, but it will return if the growth conditions remain the same.
If wood is to be painted, it is beneficial to remove mildew before painting.
This increases effectiveness of the paint film mildewcide.
are a variety of common mold species know to effect coatings performance.
A list of 19 of the most common species appears below:
sp., Aspergillus flavum, Aspergillus niger, Aureobasidium pullulans,
Botryodiplodia sp., Cephalosporium sp., Cladosporium sp., Fusarium sp.,
Helminthsporium sp., Monilia sp., Mucor sp., Pacacilomyces sp., Penicilium
sp., Pestalotia sp., Phoma sp., Pleospora sp., Rhizopus sp., Stemphylium
sp., Trichoderma sp.
these species, Aureobasidium pullulans and Aspergillus Niger are the
most common mold species encountered.
of specific species, all mildew require oxygen, water, a food source,
and a narrow temperature range to metabolize and reproduce. Typically,
mildew causes problems with the finish after it has been applied and
dried, and does not affect the product in the can as there is usually
insufficient oxygen present for metabolism. Mildew needs water to grow.
Consequently, as the humidity of the environment continues to rise,
the mildew flourishes. For food, mildew generally metabolize organic
food sources like starches, sugars, proteins, and some oils found in
paint systems. Specifically, mildew can also feed on pollens, bacteria,
or many other organic contaminants on the finished surface. Temperatures
from 70 to 90 Fahrenheit to 20 to 30 Celsius are ideal for mildew growth.
Below freezing, mildew fungi become dormant, however, they do not die.
factors that can also contribute to mildew growth include the type of
finish and its surface characteristics. Generally, top quality paints
offer the best protection from mildew. As the quality of the paint decreases,
the chance for mildew growth typically increases (a further discussion
will follow under Mildew Control). Generally, latex finishes are more
mildew resistant than alkyd paints.
growth can occur anywhere in the world. Climates that supply more of
the contributing factors discussed above will promote greater mildew
growth. For example, hot, tropical regions often have the greatest mildew
growth. Coastal regions generally grow more mildew than dry inland areas.
However, inland areas near lakes, rivers, or heavy vegetation can experience
heavy mildew growth.
mildew is already present on a substrate, the mildew must be killed
and removed before the substrate is repainted, or else the mildew will
grow through the new finish as discussed above. To kill mildew and remove
mildew from a surface, follow the steps outlined below:
Using a spray canister (one designed for insecticide application will
do) available at most local hardware stores, apply one of the two following solutions
liberally to the substrate and allow to set for about 10-15 minutes:
Solution #1 Bleach: This mixture is the old standard used for years. The chlorine kills algae, moss, and mildew. BUT - chlorine breaks down the lignin that holds wood together, causing excessive damage to otherwise healthy wood.
Chlorine is dangerous, environmentally unsound, and likely to cause damage to surrounding greenery. If you must use it we recommend
3 quarts water
1 quart common household bleach
1/4 cup maximum of liquid dishwater detergent (ammonia-free) or TSP(Tri
Solution #2 Safe & Effective:
Sodium percarbonate (Disodium Peroxydicarbonate) (not to be confused with sodium bicarbonate)
Sodium percarbonate is an excellent detergent and bleaching agent based on hydrogen peroxide. It is a good cleaning and bleaching agent at normal temperature, and has strong fungicide effect. Fruits and vegetables treated with sodium percarbonate can be kept fresh, and be stored for a long time. In medicine, it can kill staphy lococcus, and colon bacillus .
This product is a white particle powder, non-toxic no contamination, non-flammable, non-explosive, easy to get damp, and soluble in water.
Hy-Tech Oxygen bleach (sodium
percarbonate) is excellent for cleaning and removing organic stains
(such as coffee, tea, wine, fruit juices, foods, sauces, grass
and blood) from fabrics, plastics, fiberglass, porcelain, ceramics,
wood, carpets, asphalt, concrete, etc. Oxygen bleach can be used
in any place in or around the house that need to be destained
and deodorized; it is efficient, safe and economical. It is non-toxic,
environmentally safe, biodegradable, and leaves no harmful by-products
or residues which can harm the environment Except for industrial-strength
cleaning or stripping jobs, sodium percarbonate is our hands-down
choice for most average wood preparation jobs.
Mix 6-8 fl oz of Percarbonate in a gallon of warm or hot water. (2 fl oz per quart)
Bleach Safety Notes:
Do not mix bleach and ammonia. This mixture can result in hazardous,
Precautions should be taken to protect shrubs and other areas that may
be adversely affected by bleach.
Protect eyes and skin from contact with bleach solution.
the solutions to set for 10-15 minutes to give time to settle
into any crevices and hard to reach places, to kill all of the mildew
present. Skipping this 10-15 minute set time may result in an inadequate
job. The detergent is added in a small amount to help emulsify any mildew
or dirt to aid in its removal. Liquid dishwasher detergent is the best
choice because it will not foam like dry dish or laundry detergent.
Most dry detergents are not easily washed off with cold water. Use of
trisodiumphosphate (TSP) detergents is cautioned since the phosphate
may actually serve as a food source for mildew and may actually promote
future mildew growth.
4. Wash the substrate clean using a power washer. A second choice, if
a power washer is unavailable, is to scrub the surface. For masonry
substrates use a wire brush. For wood, use a softer bristle brush. For
substrates sensitive to abrasive damage like aluminum and vinyl siding,
use a sponge.
5. Use a garden hose to wash off any excess dirt, mildew, and loose
substrate residue from the surface. Residue left behind can cause adhesion
failures of the finish.
mildew was present on the original substrate or previous coats, and
a new finish coat is already applied, the mildew will grow through the
new finish. If is usually impossible to stop mildew growth at this point.
All the finishes must be stripped down to the original substrate and
then cleaned as described above before applying a new finish coat.
are chemicals added to paints and other finishes to help stop mildew
growth on the finish. There are a wide variety of mildewcides used in
the paint and coatings industry.
of these chemicals are usually listed on the container label, although
exact amounts are not usually revealed. For the consumer, attempting
to study can label analysis to determine the best mildew resistance
of a finish would be tedious and nonproductive. A majority of the mildewcide
names are extremely long, complex and meaningless to the consumer. Simply
the best way for the consumer to gauge mildew performance is by the
overall quality of the finish. Top quality finishes will offer the best
mildew protection in nearly all cases. Hy-Tech exterior coatings are formulated with the most effective mildewcides available today.
growth is an ongoing problem. Fungal spores land on surfaces and, under
the right environmental conditions, grow. Ideal conditions are warm,
moist climates, oxygen, and a substrate that serves as a nutrient source
growth on finishes causes discoloration and premature failure of the
finish. Prevention of mildew can be done by pretreating the wood with
a preservative that contains a mildewcide. Removal of mildew is achieved
by using appropriate cleaning solutions. Finishes that contain synthetic
mildewcides, with or without zinc oxide, help the finish resist mildew
Article Contributors: George Daisey, Rohm & Haas Co.; Steve Bussjaeger,
HIS Paint Mfg. Co. Inc.; Raymond Simmons, Reichold Chemicals Inc.; Saul
Spindel, D/L Laboratories; and Sam Williams, USDA Forest Products Laboratories.