Cooperative Extension Service
West Lafayette, IN 47907
Daniel L Cassens, Department of Forestry and Natural Resources. Purdue University
William C. Feist, Forest Products Laboratory. Forest Service. U.S. Department of Agriculture
Under normal conditions paint deteriorates by first soiling or by aslight accumulation of dirt. Next, a flattening stage develops whenthe coating gradually starts to chalk and erode away. Unfortunately,paint is sometimes discolored by mildew, blue stain, wood extractivesand metals long before repainting is necessary. In these cases, asimple repainting will not correct the problem for long. Furthermore,excessive painting is expensive, and a build-up of paint on the woodsurface may lead to cross-grain cracking or other severe paintfailures. If the old paint surface is not properly cleaned beforerepainting, intercoat peeling may also result.
Mildew is probably the most common cause of house paint discoloration(Figure 1). Mildew is a form of stain fungi or microscopic plantlife. The most common species are black, but some are red, green, orother colors. It grows most extensively in warm humid climates but isalso found in cold northern states. Mildew may be found anywhere on abuilding, but it is most common on walls behind trees or shrubs whereair movement is restricted. Mildew may also be associated with thedew pattern of the house. Dew will form on those parts of the housewhich are not heated and will cool rapidly such as eaves and theceilings of carports and porches. This dew then provides a source ofmoisture for the mildew.
Figure 1. Mildew is most common in shaded or protected areas.
Mildew fungi can be distinguished from dirt by examination under ahigh-power magnifying glass. In the growing stage, when the paintsurface is damp or wet, the fungus is characterized by its threadlikegrowth. In its dormant stage, when the surface is dry, it has numerousegg-shaped spores; by contrast, granular particles of dirt areirregular in size and shape. A simple test for the presence of mildewon paint can be made by applying a drop or two of household bleachsolution (5 percent sodium hypochlorite) to the stain. Mildew willusually bleach out in one or two minutes. Stain that does not bleachis probably dirt. It is important to use fresh bleach solution. Bleachdeteriorates upon standing and loses its potency.
How Paint Makeup Affects Mildew
Some paints are more vulnerable than others to attack by mildewfungi. Zinc oxide, a common paint pigment in top coats, inhibits thegrowth of mildew. Titanium dioxide, another common paint pigment, hasvery little inhibiting effect on mildew.
Considering oil-base paints, mildew progresses more readily onexterior flat house paint than on exterior enamel. Paints containinglinseed oil are very susceptible to mildew. Of the availablewater-base paints, acrylic latex is the most mildew resistant. Porouslatex (water-base) paints without a mildewcide applied over a primercoat with linseed oil will develop severe mildew in warm, dampclimates.
Mildewcides are poisons for mildew fungi. The paint label shouldindicate if a mildewcide is present in the paint. If it is notpresent, it can sometimes be added by the local paint dealer. Paintcontaining mildewcides, when properly applied to a clean surface,should prevent mildew problems for some time.
Prevention and Cure
New Wood Surfaces
In warm, damp climates where mildew occurs frequently, use a paintcontaining zinc oxide and a mildewcide for top coats over a primercoat which also contains a mildewcide. For mild cases of mildew, use apaint containing a mildewcide.
Painted Wood Surfaces
Before repainting, the mildew must be killed, or it will grow throughthe new paint coat. To kill mildew and to clean an area for generalappearance or for repainting, use a bristle brush or sponge to scrubthe painted surface with the following solution:
1/3 cup household detergent 1 quart (5 percent) sodium hypochlorite (household bleach) 3 quarts warm water*See warning later in this file.
When the surface is clean, rinse it thoroughly with fresh water from ahose. Avoid splashing the solution on yourself or on shrubbery orgrass as it may have harmful effects. Before the cleaned surface canbecome contaminated, repaint it with a paint containing a mildewcide.
In cross section, many trees contain a dark central core calledheartwood and a light colored outer band called sapwood. Dark coloredheartwood contains water-soluble extractives, while sapwood doesnot. These extractives can occur in both hardwoods andsoftwoods. Western red cedar and redwood are two common softwoodspecies used in construction. They both contain large quantities ofextractives. The extractives give these species their attractivecolor, good stability and natural decay resistance, but they can alsodiscolor paint. Woods such as Douglas-fir can also have occasionalextractive staining problems.
When extractives discolor paint, moisture is usually the culprit. Theextractives are dissolved and leached from the wood by water. Thewater then moves to the paint surface, evaporates and leaves theextractives behind as a reddish-brown stain.
Diffused discoloration from wood extractives is caused by waterwhich comes from rain and dew that penetrates a porous or thin paintcoat. It may also be caused by rain and dew that penetrate joints inthe siding or by water from faulty roof drainage and gutters.
Diffused discoloration is best prevented by following goodpainting practices. Apply a water-repellent preservative or waterrepellent to the bare wood before priming. Use an oil-based,stain-resistant primer or a latex primer especially formulated for useover staining woods. Do not use porous paints such as flat alkyds andlatex directly over the staining-type woods. If the wood is alreadypainted, clean the surface, apply an oil-based or latexstain-resistant primer and then top coat. Before priming andrepainting, apply a water-repellent preservative or water repellent toany wood left bare from peeling paint.
A run down or streaked type of discoloration can also occurwhen water-soluble extractives are present (Figure 2). Thisdiscoloration results when the back of the siding is wetted, theextractives are dissolved, and then the water runs down the face ofthe painted boards from the lap joint.
Figure 2. A streaked type of water soluble extractive discolorationcan result from water wetting the back of one piece of siding and thenrunning down on the front of the next piece.
Water which produces a run down discoloration can result from watervapor within the house moving to the exterior walls and condensingduring cold weather. Major sources of water vapor are humidifiers,unvented clothes dryers, showers, normal respiration, and moisturefrom cooking and dish-washing. Run down discoloration may also becaused by water draining into exterior walls from roof leaks, faultygutters, ice dams, and wind driven rain and snow at louvers.
Run down discoloration can be prevented by reducing condensation orthe accumulation of moisture in the wall. New houses or thoseunder-going remodeling should have a vapor barrier (continuous 6 milpolyethylene sheet) on the inside of all exterior walls. If a vaporbarrier is not practical, the inside of all exterior walls should bepainted with a vapor resistant paint. Water vapor in the house can bereduced by using exhaust fans vented to the outside in bathrooms andkitchens. Clothes dryers should be vented to the outside and not tothe crawl space or attic. Avoid the use of humidifiers. If the housecontains a crawl space, the soil should be covered with a vaporbarrier to prevent migration of water into the living quarters.
Water from rain and snow can be prevented from entering the walls byproper maintenance of gutters and the roof. Ice dam formation can beprevented by installing adequate insulation in the attic and byproviding proper ventilation. For gable roofs, vents should beprovided at the gable ends and should be about 1/300 of the ceilingarea. More positive air movement can be obtained if additionalopenings are provided in the overhang. Hip roofs should have air inletopenings in the louvers and several smaller roof vents located nearthe edge.
If discoloration is to be stopped, moisture problems must beeliminated. Run down discoloration will usually weather away in afew months. However, discoloration in protected areas can becomedarker and more difficult to remove with time. In these cases, washthe discolored areas with a mild detergent soon after the problemdevelops. Paint cleaners are effective on darker stains.
Blue stain is caused by microscopic fungi that commonly infect thesapwood of all woody species. Although microscopic, they produce ablue-black discoloration of the wood. Blue stain does not weaken woodstructurally, but conditions which favor stain development are alsoideal for serious wood decay and paint failure.
Wood in service may contain blue stain, and no detrimental effectswill result so long as the moisture content is kept below 20percent. Wood in properly designed and well maintained houses usuallyhas a moisture content of 8-13 percent. However, if the wood isexposed to moisture such as rain, condensation or leaking plumbing,the moisture content will increase, and the blue stain fungi willdevelop.
To prevent blue stain from discoloring paint, follow goodconstruction and painting practices. First, do whatever is possibleto keep the wood dry. Provide an adequate roof overhang, and properlymaintain the shingles, gutters and downspouts. Window and doorcasings should slope out from the house, thus allowing water to drainaway rapidly. Use a vapor barrier on the interior side of allexterior walls to prevent condensation in the wall. Vent clothesdryers, showers and cooking areas to the outside, and avoid the use ofhumidifiers. Untreated wood should be treated with a water-repellentpreservative, then a nonporous mildew-resistant primer and finally atleast one top coat also containing a mildewcide. If the wood hasalready been painted, remove the old paint and allow the wood to drythoroughly. Apply a water-repellent preservative, and then repaint asdescribed above.
A 5 percent sodium hypochlorite solution (ordinary householdbleach) may sometimes remove blue stain discoloration, but it is not apermanent cure. Be sure to use fresh bleach since its effectivenesscan diminish with age. The moisture problem must be corrected if apermanent cure is expected.
Rust may be one type of staining problem associated with iron. Whenstandard ferrous nails are used on exterior siding and then painted, ared-brown discoloration may occur through the paint and immediatevicinity of the nail head. To prevent rust stains, usecorrosion-resistant nails. These include high quality galvanized,stainless steel and aluminum nails. Poor-quality galvanized nails cancorrode easily and, like ferrous nails, can cause unsightly stainingof the wood and paint. The galvanized heads on nails should not "chiploose" as they are driven into the wood. If rust is a serious problemon a painted surface, the nails should be countersunk, caulked, spotprimed and then top coated.
Unsightly rust stains may also occur when standard ferrous nails areused in association with any of the other finishing systems such assolid color or opaque stains, semitransparent penetrating stains andwater-repellent preservatives. Rust stains can also result fromscreens and other metal objects or fasteners which are subject tocorrosion and leaching (Figure 3).
Figure 3. Metal screens and fasteners can corrode and later discolorpaint as leaching occurs.
A chemical reaction with iron resulting in an unsightly blue-blackdiscoloration of wood can also occur. In this case, the iron reactswith certain wood extractives to form the discoloration. Ferrous nailsare the most common source of iron for chemical staining (Figure 4),but problems have also been associated with traces of iron left fromcleaning the wood surface with steel wool or wire brushes. Thediscoloration can sometimes become sealed beneath a new finishingsystem.
Figure 4. Blue-black discoloration resulting from the use of ferrousnails.
Oxalic acid will remove the blue-black chemical discolorationproviding it is not already sealed beneath a finishing system. Thestained surface should be given several applications of the solutioncontaining at least one pound of oxalic acid per gallon of water,preferably hot. After the stains disappear, the surface should bethoroughly washed with warm, fresh water to remove the oxalic acid andany traces of the chemical causing the stain. If all sources of ironare not removed or protected from corrosion, the staining problem mayreoccur. Caution should be exercised when using oxalic acid sincethis chemical is toxic.
Chalking results when a paint film gradually weathers ordeteriorates, releasing the individual particles of pigment. Theseindividual particles act like a fine powder on the paint surface. Mostpaints chalk to some extent. This phenomenon is desirable since itallows the paint surface to be self-cleaning. However, chalking isobjectionable when it washes down over a surface with a differentcolor (Figure 5) or when it causes premature disappearance of thepaint film through excess erosion.
Figure 5. Some paints or stains chalk badly and can discolor a lowersurface as they wash down over it.
Discoloration problems from chalking can be prevented by selection ofa paint with the appropriate chalking tendencies. The manner inwhich a paint is formulated may determine how fast it chalks.Therefore, if chalking is likely to be a problem, select a paint whichthe manufacturer has indicated will chalk slowly.
When repainting surfaces which have chalked excessively, properpreparation of the old surface is essential if the new paint coat isexpected to last. Scrub the old surface thoroughly with a detergentsolution to remove all old deposits and dirt. Rinse thoroughly withclean water before repainting. The use of a top quality oil-basedprimer may be necessary before latex top coats are used. Otherwise,the new paint coat will peel. Discoloration or chalk which has rundown on a lower surface may be removed by vigorous scrubbing with agood detergent. This discoloration will also gradually weather away ifthe chalking problem on the painted surface has been corrected.
BROWN STAIN OVER KNOTS
The knots in many softwood species, particularly pine, contain anabundance of resin. This resin can sometimes cause paint to peel orturn brown (Figure 6). In most cases, this resin is "set" by the hightemperatures used in kiln drying construction lumber.
Figure 6. Brown discoloration of paint from resin exudation from aknot.
Good painting practices should eliminate or control brown stain overknots. Apply a good primer to the bare wood first. Then follow withtwo top coats. Do not apply ordinary shellac or varnish to the knotarea first as this may result in early paint failure.
The following are available from the Forest Products Laboratory, OneGifford Pinchot Drive, Madison, WI 53705-2398:
- "Painting and Finishing," Chapter 16 from Wood Handbook: Wood as anEngineering Material USDA Agricultural Handbook No. 72,1987,29 pp.
- "Wood Finishing: Weathering of Wood." USDA Forest Service ResearchNote FPL-0135, revised 1975, 4 pp.
The following are available from the state Extension Services listedat the end of this publication:
"Finishing Exterior Plywood, Hardboard, and Particleboard." NCRExtension Publication 132, revised 1988, 6 pp.
"Paint Failure Problems and Their Cure." NCR Extension Publication133, revised 1988, 6 pp.
"Selection and Application of Exterior Finishes for Wood." NCRExtension 135, revised 1988, 8 pp.
"Finishing and Maintaining Wood Floors." NCR Extension Publication136, revised 1988, 8 pp.
The following is available from the Superintendent of Documents,U.S. Government Printing Office, 710 North Capitol Street, Washington,DC 20402 (order by title and stock number):
"Finishing Wood Exteriors; Selection, Application, and Maintenance."USDA Agricultural Handbook No. 647, SN #0011-000-044-50-8,1986,56pp. Price: $3.50 (subject to change without notice).
- "Exterior Finishes for Wood." Bureau of Audio Visual Instruction,Box 2093, Madison, WI 53701.
Acknowledgment is given to the Forest Products Laboratory. ForestService. U.S. Department of Agriculture. Madison. Wisconsin, forfinancial assistance in sup port of the preparation of thispublication.
Do not mix bleach with ammonia or with any detergents or cleanserscontaining ammonia! Mixed together the two are a lethal combination,similar to mustard gas. In several instances people have died frombreathing the fumes from such a mixture. Many household cleanerscontain ammonia, so be extremely careful with what types of cleanersyou mix bleach.
Use caution with wood finishes which contain pesticides. When usedimproperly they can be injurious to man, animals and plants. For safeand effective usage, follow the directions, and heed all precautionson the labels. It is advisable to wear unlined protective gloves andto cover nearby plant life when using any material containingpesticides.
Avoid spraying a pesticide wherever possible. Drift from a pesticide,applied as a spray, may contaminate the surrounding environment.
Store finishes containing pesticides in original containers underlock and key out of reach of children and pets and away fromfoodstuffs. Follow recommended practices for the disposal of surplusfinishing materials and containers.
Note: Registrations of pesticides are under constant review bythe Environmental Protection Agency and the Department ofAgriculture. Use only pesticides that bear a Federal registrationnumber and carry directions for home and garden use. Since theregistration of pesticides is under constant review by State andFederal authorities, you should consult with a responsible Stateagency as to the current status of the pesticides discussed in thisreport.
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Issued in furtherance of Cooperative Extension work, Acts of Congressof May 8 and June 30, 1914, in cooperation with the U.S. Department ofAgriculture and Cooperative Extension Services of Illinois, Iowa,Kansas, Michigan, Minnesota, Missouri Nebraska, North Dakota, Ohio,South Dakota, and Wisconsin. H.A. Wadsworth, Director, PurdueUniversity Cooperative Extension Service, West Lafayette, IN 47907.