What is Mildew?

Definition

The 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.

Effect on Appearance and Performance

Mildew 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.

Interference with Adhesion

Paints 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.

Intercoat Adhesion to Paints

When 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.

Effect of Mildew on Wood

We 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 mildewcides.

If 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.

Common Species

There are a variety of common mold species know to effect coatings performance. A list of 19 of the most common species appears below:

Alternaria 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.

Of these species, Aureobasidium pullulans and Aspergillus Niger are the most common mold species encountered.

Necessary Contributing Factors

Substrate & Environment

Regardless 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.

Paint film

Other 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.

Geographically Prone Areas

Mildew 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 Control

Surface Preparation

If 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:

1. 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
a. 3 quarts water
b. 1 quart common household bleach
c. 1/4 cup maximum of liquid dishwater detergent (ammonia-free) or TSP(Tri Sodium Phosphate)

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:
1. Do not mix bleach and ammonia. This mixture can result in hazardous, toxic vapors.

2. Precautions should be taken to protect shrubs and other areas that may be adversely affected by bleach.

3. Protect eyes and skin from contact with bleach solution.

Allow 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.
If 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.

Mildewcides
Mildewcides 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.
Identifications 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.
Summary
Mildew 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 for mildew.
Mildew 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 growth.
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.