Clear Finishes and Stains for Wood Exteriors

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Clear Finishes and Stains for Wood Exteriors

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NATIONAL RESEARCH COUNCIL

CANADA

DIVISION OF BUILDING RESEARCH

H O U S I N G N O T E N O . 1 7

by

R. E.

PLATTS

RESEARCH

(7Lz-\

Reprinted

from

Canadian Builder, Vol.

XIV,

No.

5,

May 1964, p. 28

This _{Note may be reprinted without amendment provided acknowledgement is given to Canadian Builder.}

OTTAWA,

MAY

1964

CLEAR FINISHES AND STAIMS FOR

WOOD EXTERIORS*

BY R. E. PLATTS, Division of Building Research,

National Research Council In the past, natural shellacs, waxes, raw oils and boiled oils for wood ex- terior finishes have been tried in all manner of combinations to little avail.

In recent times, these have been modi- fied with or replaced by synthetic resins. These phenolic and alkyd varnishes and epoxy and urethane coatings have often come closer to the goal than the old finishes, but they have not allowed a good clear appearance to be maintained and renewed at reasonable cost.

Using essentially the same vehicles, pigmented stains have evolved over the same period as another "natural" ap- proach to finishing exterior woods. These can be considered a treatment rather than a coating, since the vehicle penetrates the wood and leaves little film on the surface to degrade and crack. They reveal the wood grain and texture to a certain extent, add water shedding properties to the wcod, and color it enough to reduce ultra-violet attack and to provide a uniform appear- ance as the wood ages. Some consider these pigmented stains as merely a com- promise between clear finishes and paints. Others think of them as a dis- tinctly different approach that can stand on its own merits: the stains produce dark, muted colors that can last longer and can be renewed more easily and at lower costs than perhaps any other wood finish applied in the field.

Field and laboratory experience with the clear finishes and stains has become particularly extensive over the past sev- eral years, in Canada and other coun- tries. Now the performance of even the newer types can be better clarified, in answer to the many questions of build- ers and homeowners. Technical papers describing the extensive field and labora- tory test programs by the Division are to be published when exposure phases are completed.

Clear finishes

Surface films are formed by all the clear finishes. Ideally, the film would slowly erode away while retaining its bond to the entire wood surface, thus providing a good base for easy recoat- ing. Unfortunately this rarely happens. No matter how durable the film, the wood itself is apparently attacked by the sun's ultra-violet rays and by moisture penetrating the clear film. The spring wood degrades, darkens, and releases the coating. This partially explains why darker finishes and paints, which block ultra-violet rays, usually perform better than clear finishes.

The fact that the dilution of some finishes actually improves their perfor- mance on wood may also support the observation that wood breakdown is the primary factor. Perhaps the first coat penetrates further, thus reinforcing the wood and providing better adhesion. Further, it has been shown that when sidings are exposed to the weather for several days before coating, the finishes

on these behave poorly in comparison with sidings which are coated before ex- posure.

This again suggests that degradation of the wood itself by ultra-violet rays and moisture is a prime contributor to failure of the finish. Failure to coat the sidings as soon as they are applied might be a common cause of unusually early peeling in practice. Reports suggest that the addition of 4% or more of ultra- violet absorbers in the finish can help prevent breakdown of the wood, but these amounts are very costly.

Several finishes are quite prone to "brittle failure," peeling back from a small pit or a sharp edge. Most finishes fail first at the sharp edge of the "cove" in cove sidings, and to a lesser extent at the bottom edge of all sidings. It would be best to avoid cove sidings and perhaps to round the bottom edges in the manufacture of lap sidings.

Clear alkyd coatings

The alkyds give films of exceptional clarity and durability, but their perfor- mance on woods is poor. After two years, the film properties can be good but delamination between the film and the wood can occur over wide areas, resulting in a very blotchy appearance.

Examination suggests that degradation of the wood rather than the film is the major cause of delamination, as has been noted. If left for two years, wood fin- ished with clear alkyd coatings may not

*

be suitable for recoating except by scrap- ing down to the bare wood and starting afresh.

Phenolic varnishes

The phenolic varnishes appear to be the most durable of the clear coatings. The outstanding formulations of these use paraphenyl phenolic resins with abcut a "20- to 30-gallon" proportion of Tung oil. The term "Spar Varnish" sometimes indicates such a formulation, but unfortunately it is often difficult for the user to find out the formulation be- cause of indefinite terminology and in- complete labelling. C.G.S.B. Specifica- tion 1-GP-99 of the Canadian Govern- ment Specifications Board provides the above formulation, but this does not help smaller builders or homeowners since they have no way of knowing which varnishes comply. Larger builders can demand that their paint suppliers meet certain specifications.

The better phenolic varnishes can give about two years' service with few ob- vious faults. Recoating can be done after some scraping and sandpaper feathering around spots that have failed. Points of failure generally form as small craters and cracks which extend through to the wood. If more than two years is allowed to pass before refinishing, the film can peel back from such cracks and removal of the entire film may be required before recoating.

Varnishes have a tendency to turn yellow when exposed, becoming darker with time and with recoating. Because of this darkening, and the presence of cracks and craters, it is doubtful if many would be satisfied with their appearance after one or two recoatings. This means that the best varnishes are limited to about two years before some scraping and a new coat is required, and perhaps up to six years before complete removal and a new application is desirable.

Epoxy finishes

The epoxy esters have generally per- formed very poorly. Some two-part epoxies are somewhat better, with dur- ability comparable to the alkyds. Unlike the alkyds, the film itself deteriorates and flakes extensively within two years, so that recoating is not possible without complete removal.

Polyurethanes

The results with these new, widely acclaimed finishes are both disappoint- ing and encouraging. The oil modified polyurethane varnishes show great varia- tion between different areas of wood with performance ranging from very bad to good. These urethanes seem to pro- duce a film with high internal stresses which cause it to peel back rapidly from any point of rupture.

Some two-part urethane resins also

RACKS LIKE THESE are used at the Division of Building Research in Ottawa to check the durability and other quality of finisher under exposure.

show brittle sensitivity to small points of rupture, but in general they can per- form quite well. They may be the only clear finish that shows little failure be- tween the finish and the wood, and so appear very suitable for recoating with little preparation.

Unfortunately, the new coats can fail quite quickly. High internal stresses plus a lack of adhesion to the first urethane coat, can cause them to peel back over large areas. Perhaps the flex- ibility of this type of two-part urethane can be further improved by adding more plasticizers, and adhesion might be im- proved by other means, The urethanes should be the most promising of all the clear finishes if the recoating problem can be solved.

Stains

Pigmented stains of two types, modi- fied oils and alkyds, appear to perform better than any other natural finish. A third type, with a refined creosote base, often shows some leaching by rain re- sulting in a somewhat spotty appear- ance. Recoating is easy and the appear- ance is much improved, but is some- what uneven. Further, creosotes are a risky base if painting is later desired, since they tend to discolour light-color- ed oil paints.

One of the older, natural oil type of stains is apparently still one of the best available finishes. This is the U. S. Forest Products Laboratory stain devel- oped in the early 1950's. It produces fairly dark tones, masking the wood grain to a considerable extent but re- vealing its texture with a low sheen appearance. A single coat can look well for 5 years and more, even on smooth cedar sidings. The finish weathers slow- ly in a desirable manner, by erosion, leaving an excellent base for restaining. Since these stains contain a wax, they should be weathered at least one year

if it is desired to paint over them. Alkyd stains generally meeting the re- quirements of C.G.S.B. Specification 1- GP-145, Type I, can apparently perform as well as the U. S. Forest Products Laboratory stain. These stains may be satisfactory for several years, and should also perform well after recoating. The alkyd formulations produce lighter colors than the U. S. Forest Products Laboratory stain, but are still quite opaque and mask much of the wood g a i n . Perhaps less pigment or lighter- colored oxide pigments can be used to allow a wider choice of colors with the stains, and still provide a durable, at- tractive treatment.

What field experience

shows

Field experience shows that vertical siding behaves better than horizontal as a base for stain since the grain runs in the direction of water shedding. The bottom ends of the boards should have the end grain liberally filled with the stain.

It has also been observed that rough- cut sidings hold the stain better than planed surfaces. Rough cedar shakes have been observed in attractive condi- tion 10 to 15 years after the stain was applied. Builders are now often using rough sawn vertical sidings with dark stains, board-on-board or board-and- batton. It may well be that these will cause the last trouble with the lowest annual costs of any cladding or veneer on the market.

Building research

The Division of Building Research of the National Research Council has is- sued a report on the work of the Divi- sion for the year 1962. It continues an over-all review of the Division's work presented in previous a ~ u a l reports. Copies are available at $1.00 each.