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Technical Note (National Research Council of Canada. Division of Building Research), 1966-04-01
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Paint Performance on Wood Sidings in the Atlantic Provinces
Tibbetts, D. C.
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EXTERNAL FACTORS CONTRIBUTING TO PAINT FAILURE NBH ,
No.
465 April 1966 APPROVED BYNOTlE
CHECKED BYDIVISION OF BUILDING RESEARCH
']['EClHI N
J[
CAlL
D. C. Tibbetts
This report deals with paint performance on wood substrates with particular reference to sidings, where the greatest incidence of paint deterioration occurs. Both paint and wood are complex materials and water plays a dominant role in this deterioration. Temperature changes and the effects of ultraviolet light cannot be overlooked since field observations indicate that south, west, and east exposures are the most affected and a combination of wind-driven rain and sunlight may be involved.
In houses, moisture may gain access behind painted sidings and trim by condensation during cold weather of vapour originating within the house. It may also enter from the outside by penetrating through joints. The latter is probably the more common cause, but it is often overlooked because cold weather condensation has been so widely publicized. It is the intention of this Note to explore the external causes while recognizing that cold weather condensation may play an additional role in the deterioration of paint films on exterior wood
substrates.
1. Construction defects.
2. Excessive inside relative humidity (R. H.). 3. Ic e damming.
SUBJECT PAINT PERFORMANCE ON WOOD SIDINGS IN THE ATLANTIC
PROVINCES
PREPARED BY
4. Poorly cured siding. 5. Workmanship.
6. Paint quality.
7. Atmospheric conditions during application. 8. Exposure.
Construction Defects
Construction defects often contribute to paint failure. The following details of construction are recommended as aids in over-coming such failure s.
(a) Proper structural design is needed to keep wood dry enough to hold paint. Such de sign would logically include proper choice of wood type and would protect against the entrance of moisture behind painted wood surface s.
(b) Flashing around dormers, chimneys, roof valleys, and over doors and windows should extend at least 6 in. under roof shingle sand siding.
(c) Squared ends should be set in paint to prevent admittance of water. (d) Joints should be tightly sealed to prevent any leakage through theIl':. (e) Window heads and siding butts should be well fitted.
(f) Siding should never touch the damp ground.
(g) Wide overhang at gables and eaves in the Atlantic region offers additional assurance against rainwater penetration of walls.
Paint peeling is more prevalent on bevelled wood sidings than on other wood sidings and frequently has been observed to start near the ends of such siding where two piece s butt on a wall, next to corner boards and window details, and in the case of vertical siding, at the butts over window heads and at flashed water tables.
Excessive Inside Relative Humidity
Although of secondary interest for purposes of this report, it is considered good practice to provide adequate vapour barriers as well as making provision for adequate ventilation of interior -derived water vapour. There is sufficient field evidence to suggest that in the modern, insulated, and tightly sealed house there are many examples of paint failure that cannot be attributed to water vapour build -up. Sidings on many unheated, uninsulated, and well-ventilated buildings are frequently subject to peeling.
Ice Damming
Ice damming is very common in some parts of the region. Numerous cases have been noted where generous eave overhang in
one part of the region has provided protection to the painted siding on the upper part of the wall while, owing to ice damming in other areas, water has gained access to the walls and soffits causing severe deterioration. In some instances, ice forming at the eave has caused water to back up under roofing shingles and from there it has run down through the walls behind the siding with resultant failure. The most effective corrective measures appear to be to use wide rolled roofing or sheet metal under the first few courses of eave shingles to prevent water from entering the wall and also to provide proper ceiling
insulation, coupled with eave ventilation, to keep the roof surface relatively cool and so reduce the melting of snow and ice that creates ice dams on roofs.
Poorly Cured Siding
Excessive moisture in wood is considered to be the most common cause of paint blistering and peeling; the cure is to eliminate the source of the moisture. Free water in the wood is necessary to initiate the mechanism of blistering - water vapour is not sufficient. Failures that do occur are usually confined to only a part - - often a very small part - - of the whole painted area. This suggests that some conditions are different on those areas where failure occurred. Green wood is reportedly more subject to paint failure than dry wood that may have been exposed to rain and dampness immediately prior to painting. It may be that, before being painted, wood sidings should be allowed to adjust to the equilibrium moisture content for the particular region and species involved.
Workmanship
Usually different paints and painting procedures should be used for different substrates. The adherence and performance obtainea
, , 'I I
from the same paint applied to different substrates (woods) rrl.ayJv'ar
y
, I
appreciably. Surface preparation is as important as actual pain', ' application, if not more so. The greatest difficulty appears tp of cur when houses are repainted where, in many cases, no serious atteFpt ' is made to remove dirt and loose paint before the new paint is applied. One important shortcoming thctt can be attributed to workmanship is , the adulterating of factory-prepared primers. Painting is too frequently
"
thought to be the type of work that "anybody can doll and not enough time or attention is given to supervision and inspection.
Paint Quality
Claims of improved quality and durability of paint films have been made in recent years in spite of the fact that painting practice has in some cases been reduced from three to two-coat applications. Intact films with up to six year s· exposure have been reported but observations in the field indicate that four -year repainting cycles are more common. Incompatibility of some paints with previous coats is frequently cited as a cause of failure. Generally, compatibility of coatings is not a problem when they are all of the same generic type. It is usually when different generic types are involved that compatibility problems occur. Paints of high permeability to moisture vapour, such as most latex paints, theoretically resist blistering by letting moisture escape from the paint-wood interface as fast as it gets there. Paint of good quality conforming as a minimum to currently available govern-ment specifications should be used and applied in accordance with the manufacturer· s recommendations.
"There is hardly anything in the world that some man cannot make a little wor se and sell a little cheaper, and people who consider price only are this manls lawful prey." -- John Ruskin.
It is frequently stated that today· s paints are "better than ever"; the fact remains, however, that paint failures still occur and that the phenomenon of peeling appears to be little understood.
Atmospheric Conditions During Application
Paint should not be applied during periods of heavy dew, fog, precipitation, low temperatures and excessively hot temperatures follow the sUn around. The length of time for drying is extended during periods of high relative humidity; some thought should be
given to the optimum curing time and conditions for paint as for concrete.
Exposure
With some exceptions it is natural for the paint to "weather away" earlier on the southern and eastern exposures. Five years is generally regarded as a reasonable life possibility for a paint of good quality on exterior surfaces, but this is naturally influenced considerably by the conditions of exposure. For example, disintegration is more rapid
on the southern aspects of a building than the northern, owing to greater variations of teIllperature, etc. Wall surfaces to the south, east, west and north will probably require refinishing in that order, dependeht upon geographic location and erosion of the finish by the weather. The lower rows of siding Illay need refinishing before the protected siding located closer to the eaves, especially where eave overhangs are provided. Soffit areas sometiIlles fail due to excessive paint build-up by repainting before previously-applied paint has
weathered. PAINT FILMS
Paint as an applied material or filIll is quite thin with the recoIllIllended desirable thickness being 0.0045 to 0.0055 in., and is influenced by solar radiation, high teIllperatures, low teIllperatures, Illoisture frOIll rain, dew or condensation, atIllospheric pollution, and Illould growth.
Field observations revealed that earlier and Illore extensive deterioration of paint filIlls occurs on southern and eastern exposures than in applications exposed iIi. other directions. Also, on northern exposures, where sunlight is less intense, sOIllewhat longer durability Illay be obtained. In addition to the deteriorating effects of ultraviolet light it is COIl1Il1on in the Atlantic region for paints exposed to the
south, southeast and east to receive the greatest aIllount of wind-driven rain. It is not uncoIllIllon to find that the north side of a painted wooden building in this region is in good condition, while deterioration has occurred on the other exposures.
The norIllal process of decay on exposure to weather is due to the continual and varying action of changes of teIllperature, Illoisture, and sunlight. The destructive action of these natural agencies is ｧｲｾ｡ｴｬｹ
increased by the iIllpurities present in the atIllosphere of towns, particularly in industrial areas where cheIllical fUIlles are prevalent. SOIlle suspected
reactions of hydrogen sulphide and lead oil paints have been observed in the region. Sea air also has a decided tendency to accelerate decay as do conditions of warmth and moisture conducive to Illould growth. '
It can be stated generally that the life of the paint, which dE1pends on its resistance to these destructive agencies, is influenced both by the cOIllpositions of the IllediuIll and that of the pigIllents incorporated in it.
WOOD
Successful painting depends to a great extent on the species and grade of timber, the extent to which it has been seasoned and the way, in relation to the grain, in which the timber has been sawn from the log. Added to this is the degree of surface finishing, i. e. planed
surface or sawn surface.
The amount and distribution of summerwood in relation to springwood have a marked effect upon both the appearance and the
durability of paint. Problems arise due to differences between the thin-walled larger cells in springwood and the thick-thin-walled smaller cells in summerwood. More oil will be withdrawn from the priming paint by absorption within the summerwood fibres than by the springwood. This may unduly upset the ratio of pigment to oil on which the adhesion and ultimate durability of the paint depend.
Woods behave differently when moisture is gained or lost according to the difference between the summerwood and springwood of the annual growth rings and to the amount and distribution of summerwood relative to springwood. The swelling of summerwood due to moisture absorption may be greater than that of the springwood; in fact the latter may be compressed and actually shrink while the summerwood is expanding. The se dimensional change s in the wood fibre will tend to weaken the adhesion of the attached paint film. This is possibly one of the principal causes of the well-known type of paint failure on wood exposed to the weather where the paint gradually cracks along the grain, then curls away from the wood and subsequently flakes of£.
It is taken for granted that softwoods are the most suitable as paint-holding woods for exterior surfaces, and that durability of paint generally depends on the density of the wood upon which it is applied, being best for woods of low density. It is not always recognized, however, that even within a wood species, the density may vary sufficiently to affect seriously the paint-holding power of individual boards. The manner in which'wood grain is exposed by sawing affects the ability of paint to adhere to the wood and thus affects ultimate
durability. It has been said that circular - sawn lumber checks considerably more than does band-sawn. A classification of softwoods for painting is given in Table 1.
Bevel siding should be edge grain only. The use of rough-textured sawn faces for siding is becoming more common but even when this is done, the edge grain is best because it will lie flatter. When the sawn face of flat grain bevel siding is turned out, however, equally good
results will be obtained from the "bark" face and the "heart" face. An important consideration for successful painting on flat-grain or mixed-grain siding is that the pattern side or surface to be painted be run on the "bark" face. This means that the troublesome "heart" face will be turned to the back against the wall.
Adhesion of the paint film to the surface is more important than penetration into the surface. As a general rule, paint will peel from summerwood but will adhere satisfactorily to springwood. (Summerwood may be three times as dense as springwood.) Paint lasts longer on edge-grain boards than on flat-grain boards. Although summerwood has only 1/2 the capacity of springwood for holding
liquids, liquids generally move through summerwood much more readily than through springwood. Paint oils penetrate deeper into the summerwood.
The prime coat of paint applied to new wood is very largely taken up by the cavities of wood cells that were cut open in the planing process. Rough surfaces such as the sawn side of bevel siding al,so consume a great deal of paint, as do weathered boards if they are painted after they become rough. After weathering all wood seems greedy for paint and no relation can be discovered between spreading rate and the density of the wood. Moisture content below the ｶｩｳｾ｢ｬ･
saturation point of wood does not render wood unsatisfactory for' painting.
The extractives in some woods are thought to have an influence on paint performance. Certain kinds of cedar contain oils that tend to interfere with the drying, hardening and proper adhesion of paint. Water-soluble substances, in some woods, may cause discoloration of white paint. The major component of the material leached from Western Red Cedar by water is an exceedingly reactive organic acid named
Plicatic. As Plicatic acid is nonvolatile and soluble only in water,' the only way it can reach the paint surface is when it is in solution in water. Thus the staining problem is clearly a moisture movement problem. The resin in pine lumber slightly shortens the life of paint containing zinc o:xide, but this effect is corrected by using paint free from zinc oxide.
PRIMERS
Too much importance cannot be attached to the efficiency of the priming; without proper priming the protection afforded by subsequent coats cannot be relied upon.
As colour, colour maintenance, cleansing, re sistance to deterioration by ultraviolet light, and mildew resistance are of lesser importance in a primer coat, those properties that impart to the paint film greater resistance to blistering can be concentrated on.
For most exterior grc,.de softwoods, a good quality white lead linseed oil primer, or similar material containing about 10 per cent of genuine red lead, should be used. The main exception to the use of a lead primer is on Douglas Fir because the paint is prone to crack along the grain. For Douglas Fir, and in certain instances lower grades of other softwoods, an aluminum priming paint made from good leafing aluminum powder and a highly water-resistant varnish, is recommended. Good properties depend not only on the leafing
characteristics of the aluminum powder, but also on the composition and character of the varnish medium which is usually made from a phenolic-type synthetic resin and tung oil with or without a certain amount of linseed oil. In particular, a house paint primer should be free from zinc oxides because primers without zinc oxide show
minimum swelling. The primer and finish coats should be of the same brand.
It is preferable to give end grain two coats of primer in order to prevent entry of moisture which may subsequently lead to failure. Contact surfaces of all members should be primed before joining them together.
FINISH COATS
The first, and vital, function of a paint film is to protect the wood surfaces from the effects of weathering by excluding moisture from the wood and providing a shield against the influence of light on the structure of the wood. This protection is usually obtained by
suitably pigmenting a binder such as linseed oil. The pigments exclude light from the wood and the binder protects the wood from readily
absorbing moisture. Finish coats should be of the same brand as the primer. There are many finish coats available; they differ so widely that no one primer can be expected to be compatible with all of them.
During the fourth quarter of the nineteenth century four coats were considered necessary for painting new exterior woodwork. By World War I three coats had become accepted practice. By 1940 the two-coat system became established and generally accepted. For two-coat work to be satisfactory, the paints must be properly designed
for application in thick coats. Between 1926 and 1931, the U.S. Forest Products Laboratories in Madison, Wisconsin, conducted a series of exposure tests which indicated that durability depended On the thickness of the coating regardless of the number of coats, the desirable thickness for durability apparently being between 0.0045 in. and 0.0055 in. The test results also revealed that paint systems with relatively low swelling rates in water have the best chance of re sisting failure on wood surface s"
The most favourable period for exterior painting is in the late summer and fall as the surfaces are usually dry and good penetration results so that the protective value of the film is
enhanced and de struction through premature exposure to the summer sun is avoided.
Most cases where finish paint peels from the undercoat are due to application over damp surfaces" Although painting during rainy periods is usually avoided, too little attention is given to the harmful effects of moisture films or dews present on surfaces in the early morning where the house is shielded by the eaves, and to the fact that on parts not exposed to the morning sun surface moisture may per sist for a considerable time causing poor adhesion between coats.
1
I
In former tlmes most reSl entla palntlng was. . "d " l . . done Wlt. h
I
a.
.
type of paint with outstanding ability to stand neglect well. Later ｴｨ･ｾpopular choice shifted to types that have certain important adva:r1,tages during the useful life of the coating but do not stand neglect ｷ･ｬｬｾＱＧ
r
Certain repainting combinations that have repeatedly been observed to cause abnormal and usually unsatisfactory developments are white paints over coloured pigment paints such as red, brown. green or black; almost any house paint over clear varnish; pure ' white lead over mixed pigment paints that have not aged for a very long time; enamelized paints over softer paints of the ordinary types
l;
and ordinary paints over enamelized paints" A properlyｦｯｲｭｵｬ｡ｴｾ､
j,dark-coloured house paint is reportedly very durable on exterior
ｷｯｾ
surfaces. Mixed pigment paints can usually be applied with safetyt
over white lead paint but many cases of coarse alligatoring have been reported when white lead paint is applied over paints that contain zinc oxide. There is no one type of paint that can be applied over all other types with assurance that it will perform well.The average life that can be reasonably expected from a field-applied coating today is four to five years. To increase the average life of exterior paint systems by one year would be a major achievement.
A review of the literature indicates that ｾｮ painting on wood: Paint adheres better to softwoods than to hardwoods.
Performance of paint is better on some species within the softwood varieties than others.
The density of the wood has an influence - - low density is better than high density.
Spring wood is better than summerwood. Bark side is better than heart side.
South and east sides of buildings deteriorate faster than the other exposures.
Quarter sawn (edge grain) is better than flat sawn (flat grain). Sawn surfaces are better than planed surfaces.
Zinc-oxide types should not be used as primers. Architectural details, e. g., flashings are important. Some aspects of condensation have been over publicized. Expansion and contraction of paint films are quite different than those occurring in wood.
The ratio of summerwood to springwood has an influence on performance.
Adhe sion of paint to the surface is more important than penetration.
Extractives in wood have an influence on paint performance. Primers and finish coats need to be compatible.
What the construction industry wants - - a miracle product that: Can give perfect coverage on any wood product, regardless of the moisture content, sap content, grain, checking, or any other condition encountered in lumber.
Will dry rapidly, evenly, and have perfect adherence regardless of the weather or site conditions.
Can be applied by any skilled or unskilled mechanic either by day or by night.
Will always look the same on a large wall structure as it looks on the small colour chip.
Can be applied on a small area where the painter skipped a brush, or where the ladder has scraped the paint, or where minor repairs to the siding are made, and blend completely with the rest of the paint job.
(6) Will last a minimum of ten years and can be repainted with one coat which will produce a "like -new" finish.
(1) (2) (3) (4) (5) (6) (7) (8)
(9)
(lO) (11) (I2)e
(I3) (I4) ( 15) ( 16) (1) (2) (3) (4) ( 5)JOINT RESEARCH PROJECTS (Wood and Paint Industries)
(l) Conference on exterior coatings for wood sponsored on 17 April 1963 by the Canadian Paint, Varnish and Lacquer As sociation and the Canadian Wood Council at which the need for better communication between the two industries was pointed out and the hope expressed that the conference would produce a working liaison organization.
(2) As reported in the Canadian Builder, August 1963,
representatives of the U. S. A. Paint and Lumber Industries voted to continue in 1964 a $50,000 joint research project designed to improve the performance of painted wood. Research into ways of increasing the longevity of exterior wood finishes is being carried out by the U. S. Forest Products Lab. and the Armour Research Foundation. A project which will attempt to compile statistical infor-mation on the number and causes of unsatisfactory paint performance on tract housing has also been initiated. BIBLIOGRAPHY
(l) Field Applied Paints and Coatings. Building Research Institute, Washington, Publication 653, 1959.
(2) How to Paint Your Wood Home. A publication developed by a committee of the National Lumber Manufacturers Association and the National Paint, Varnish and Lacquer Association,
February 1963.
(3) Painting Woodwork. Building Research Station Digest No. 30 (Series 1), May 1951.
(4) Painting Hardboard. Canadian Paint, Varnish and Lacquer Association, September 1958.
(5) The Painting of Buildings. Post-War Building Studies No.5, London, 1944.
(6) Estrada, N. S. Exterior Finishes for Wood - - Some of the Problems Involved. Forest Products Journal, June 1959.
(7) The Two-Coat System of House Painting. Report No. 1259. U. S. Forest Products Laboratories, September 1958.
(8) Reno, John. Trouble-Free Siding. Palco Redwood. (9) Forest Products Journal, June 1960. p. 7A.
(10) Technical Bulletin ｎｯｾ 118-55 of the Western Red Cedar Lumber Association.
(11) Reno, John. How to Select and Apply Natural Finishes for House Sidings. Retail Lumberman, February 1957.
(12) Kuzmak, J. M. and P. J. Sereda. The Blistering of Paint in the Presence of Water. National Research Council, Division of Building Research, Research Paper No. 13, January 1955 (NRC 3453).
(13) Mack, G. W. Why Paint Fails. British Building Research Station.
(14) Browne, F. L. Understanding the Mechanisms of Deterio-ration of House Paint. Forest Products Journal, November 1959.
(15) Suggestions on Overcoming Construction Defects and Other Factors That Cause Paint Failure on Wood Surfaces. Circular 723, Scientific Section, National Paint, Varnish and Lacquer Association Inc., Washington, 1947.
(16) Reno, John. A Work Sheet--How to Locate Causes of Paint Blisters and Stains on Siding. Palco Redwood.
(17) Brown and Laughnan. A New Device for Studying the Blister Resistance of House Paints. Forest Products Journal, March 1960.
(18) Veer, J. J. G. Blistering of Paints on Wood. Lumber Dealer and Buyer, August 1957.
(19) Painting, Repairs and Utilities. TM5-618, U.S. War Department Technical Manual, October 1946.
(20) Wood Properties and Paint Durability. Misc. Pub. 629, U. S. Dept. Agriculture, Forest Service, June 1962.
(21) 7th Annual Progress Report, Rhop1ex AC33 and AC55 100% Acrylic Emulsions for Exterior Paints. Rohm and Haas.
(22) Browne, F. L. Repainting the Paint-Neglected House. Report No. R1135, U. S. Forest Products Laboratories, 1935.
(23) Browne, F.L. Behaviour of House Paints on Different Woods. No. 1053, U. S. Forest Products Laboratories (revised April' 1948) .
(24)
•
Browne, F. L. When and How to Paint Homes and Farm Buildings. Report No. R962, U; S. Forest Products Laboratories, (revised May 1938).(25) Conference on Exterior Coatings for Wood. Canadian Paint, Varnish and Lacquer Association and the Canadian Wood Development Council, April 1963.
•
e
TABLE I
CLASSIFICATION OF SOFTWOODS FOR PAINTING
e
Group 1 Hold paint longest; suffer least from weathering
Western Red Cedar Redwood
Group 2
Hold white lead as well as Group 1 but do not hold mixed-pigment paints as long; more easily weathered Northern White Pine
Group 3 Hold paint poorly; weather easily Eastern Hemlock Western Hemlock Eastern Spruce Commercial White Fir Group 4 Have poorest paint-retaining
characteristics
Douglas Fir
Norway (Red) Pine Tamarack
Hardboard is without grain and, as a result, has substantially equal strength in all surface directions and has neither knots nor other natural imperfections. Also, any slight
dimensional changes would be uniform in all surface directions. The se factor s are advantages with respect to paint-holding characteristics.
