This report describes briefly a series of experiments made at the Forest Products Laboratory, Madison, Wisconsin, to determine the comparative moisture resistance of linseed oil, impregnation treatments, condensation varnishes, oil varnishes, enamels, cellulose varnishes, rubber, electroplated and sprayed metal coatings, and metal-leaf coatings when applied to wood. All coatings except rubber and electroplated metal coatings, which were not developed sufficiently to make them practical, admitted moisture in varying degrees. The most effective and most practical coating was found to be that of aluminum leaf

Dunlap, M E

English

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REPORT No. 85 
MOISTURE RESISTANT FINISHES FOR 
AIRPLANE WOODS 
; 
AIRCRAFT TE;;H!lICP.L tJQ~. On I Y ~ 
NATIONAL ADVISORY COMMITTEE 
FOR AERONAUTICS 
lit..) LJoJCtJMErIT LO;';; mo:., T:IE FILES CF 
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mQl [SIS f~. UOL':ATIO S SHOULD BE ADDRESS£,,[) 
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17::'4. S""Ff.IT. .W., 
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WASHINGTON 
GOVERNMENT PRINTING OFFICE 
1920 
I 
https://ntrs.nasa.gov/search.jsp?R=19930091148 2020-06-17T04:34:15+00:00Z
REPORT No. 85 
MOISTURE RESISTANT FINISHES FOR 
AIRPLANE WOODS 
NATIONAL ADVISORY COMMITTEE 
FOR AERONAUTICS 
WASHIN GTON 
GOVERNMENT PRINTING OFFICE 
1920 

REPORT No. 85 
MOISTURE RESISTANT FINISHES FOR 
AIRPLANE WOODS 
BY 
M. E. DUNLAP 
Architectural Assistant in Forest Products 
/ 
3 
FOREWORD. 
This publication is one of a series of eight monographs prepared by the Forest Products 
Laboratory of the Forest Service, United States Department of Agriculture, for publication 
by the National Advisory Committee on Aeronautics. 
The purpose of the series of monographs is to discuss in detail the various requirements of 
wood for use in aircraft and to make public some of the results obtained in the experimental 
and testing work of the Forest Products Laboratory undertaken for the Army and Navy during 
the war. 
The subjects discussed will include: (1) Kiln-drying of airplane woods, (2) the effect of 
kiln-drying on strength, (3) the care of airplane stock, (4) the composition and use of glues, 
(5) the manufacture and testing of plywood, (6) wood in airplane construction, (7) moisture-
resistant finishes, and (8) wood airplane parts. 
4 
REPORT No. 85. 
MOISTURE-RESISTANT FINISHES FOR AffiPLANE WOODS. 
By M. E. D UNLAP. 
In an investigation made at the Forest Products Laboratory to :find an airplane-propeller 
coating which would prevent moisture changes in the propeller, a great many varnishes, enamels, 
and other coatings were studied. The study was confined to the ability of the coatings to pre-
vent the passage of moisture either into or out of the wood. Other problems, such as the dura-
bility of the finish and how to apply stain, were not studied. 
This report deals with the various coatings which were tested and their effectiveness as 
moisture-proofing agents. The terms (( moisture proof" and ((moisture resistant" indicate 
resistance to the passage of moisture through the coating. They do not refer to resistance to 
discoloration in the presence of moisture. 
METHOD OF TESTING. 
The tests were made by applying the coatings to panels of yellow birch % by 4 by 8 
inches in size. The surfaces of the panels were carefully smoothed and the corners were rounded. 
In general, a coat of :filler was first applied, followed by three coats of the varnish or other 
material which was being studied. Some materials which were studied required special methods 
of application, depending upon their character. 
After the panels had dried thoroughly they were subjected for 17 days to an atmosphere 
with 11 humidity of 95 to 100 per cent. The absorption of moisture during this period, calculated 
in grams per square foot of surface exposed, was taken as a measure of the effectiveness of the 
coating. 
MATERIALS USED. 
In general, matetials of the following types were tested: Oil, wax, oil varnish, enamel, 
spirit varnish, cellulose varnish, condensation varnish, rubber coating, and metal coating. 
liinseed oil and wax treatment.-The first specification issued by the Air Service for coating 
propellers required five coats of linseed oil applied hot and two coats of floor wax. This coating 
reduced the absorption of moisture only \"ery slightly. 
Impregnation treatments.-Test panels were treated by forcing various material, including 
varnish, china-wood oil, and cellulose varnishes, into the wood. These treatments were made 
by immersing the specimens in the liquid in a treating cylinder and drawing a vacuum until 
a considerable amount of the air had been removed from the wood. The vacuum was then 
released and atmospheric pressure was allowed to force the liquid into the wood. Specimens 
were usually allowed to stand in the liquid overnight, after which they were removed, wiped 
off, and dried. The moisture resistance of the wood was only slightly increased by these 
treatments. 
Condensation varnish.-Considerable time was spent in experimenting with this material. 
It is a patented product made from formaldehyde and phenol and is usually thinned with 
alcohol. It is extremely hard to apply as a coating because of the rapid evaporation of the 
thinner and because it must be baked for several hours at a temperature of about 180 0 F. 
The resulting coating was not very effective on account of holes, was extremely hard and 
brittle, and if it became broken it was subsequently further cracked by the swelling of the wood 
through absorption of moisture. This type of coating was given up as entirely unsuitable. 
Crazing was also a fault of this material when used as a varnish. 
5 
6 ANNUAL REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS. 
Oil varnish.-In panels made by applying three coats of varnish over a filler, the reduction 
in moisture absorption of the natural wood averaged about 66 per cent for 43 spar varnishes 
and about 85 per cent for 3 rubbing varnishes. 
Enamels .-These are varnishes to which pigment has been added. Some interesting 
points were brought out in this test. It was found that the fl,ddition of a pigment to a spar 
varnish added materially to the moisture resistance of the coating. This was true of aU pig-
ments which were tried, including white .lead, red lead, orange mineral, and barytes. The 
pigment barytes showed great superiority over the others tested. It has poor covering proper-
ties, and where equal parts of barytes and varnish by weight are mL~ed and ground together 
the transparency of the coating is little affected. This coating prevented the absorption of 
about 92 per cent of the moisture which would be absorbed by natural wood under the same 
conditions. 
In this connection it might be mentioned that if a varnish be allowed to dry almost dust 
free, and then be rubbed with aluminum powder, a slight improvement in the moisture resist-
ance of the coating will be obtained. Aluminum powder added to the varnish itself will also 
produce beneficial results. 
Oellulose varnishes (pyroxylin).-A number of tests were made on these materials, although 
they are not commonly used for wood. In ability to prevent moisture absorption they fell 
in the same class as spar varnishes. The mo t successful way to apply them is by means of 
dipping machines or air bru hes. 
Rubber coatings.-Some tests were made of hard-rubber coatings which were applied to 
wood and vulcanized. The coating was found to be moisture proof. It did not adhere well 
enough to the wood, however, to pass whirling tests applied to propellers. 
Sprayed metal.-The Schoop proce s of coating wood with molten metal was studied, u ing 
aluminum and copper. It was found that the coating produced was not perfect and did not 
adhere to the wood well enough for the purpose intended. The coating is also quite heavy. 
This mcthod is used ordinarily in ornamental work. 
Electroplated coatings.- In this case a moisture-proof coating was obtained. It was found, 
however, that the coating showed little strength, did not adhere sufficiently to the wood, and 
was quite heavy. For the e reasons it appeared to be entirely unsuitable for use on propellers. 
o information is available concerning the method used in applying this coating. It appears 
that a coating of finely powdered copper was applied over the surface of the wood, which was 
then placed in a bath and the coating of copper applied electrolytically. 
Metal-leaf coatings.- The most success;ful practical coating found consisted of a varnish or 
enamel coating in which aluminum leaf was incorporated. 
Two types of aluminum-leaf finishes were used, which are about equally resistant to 
moisture transmission. One makes use of spirit varnish and the other of oil varnish, the 
successive coats being as follows: 
Spirit-varnish type.-Fi.ller, :plus 1 or 2 coats orange shellac, plus 1 coat spar varnish 
used as a size, plus 1 coat aiumlllum leaf, plus 2 coats of orange shellac with desired 
color, plus 1 coat par varnish. 
Oil-varnish type.-Filler, plus 1 or 2 coats spar or rubbing varnish, plus 1 coat 
spar varnish used as a size, plus 1 coat aluminum leaf, plus 2 coats spa.r varnish or 
enamel . 
Coating wood with metal leaf is not nearly so slow a process as laying leaf in sign making. 
As soon as the size reaches the right condition, the leaf can be applied b,!}" unskilled workmen 
directly from the book without the aid of gilders' tips. The time required to apply leaf to a 
propeller should not be more than 40 or 50 minutes, and this could be reduced as the finisher 
becomes more experienced. 
It is important to allow the size to reach the proper condition before attempting to lay 
the leaf; the right point is just before the varnish sets dust free. The time required after appli-
cation to reach this condition varies with the type of varni h used, but for spar varnishes it 
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MOISTURE RESISTANT FINISHES FOR AIRPLANE WOODS. 7 
is usually from 1 ~ to 2 hours. The workman will soon learn how to judge the condition of 
the size by touching it lightly with his fingers . The size will dry much more quickly if it is 
thinned about one-fourth with tUl'(wntine. It should be applied as sparingly as po sible. 
To apply the complete finish of the spirit-varnish type requires in Lhe neighborhood of 
10 hours and to dry the various coat about 90 hour, making the total time about 100 hours. 
The oil-varnish finish takes longer to dry and would probably total about 240 hours. The 
latter finish is possibly the more durable. 
Different numbers oj coats oj varnish.-An interesting series of experiments was carried out 
by applying different numbers of coats of varnish (from 2 to 12) to standard test specimens. 
These tests were made in such a way that all panels received their final coat on the amc day. 
Two spar varnishes of medium moisture r esistance and one of good moisture resistance were 
used . The two varnishes of medium grade gained in re istance as the number of coats incrcased 
up to about six coats; after this point there was practically nothing gained by the addition of 
subsequent coats. In the ca e of the more resistant varnish an increase in moisture resistance 
was observed with the increased number of coats up to 12, although the gain became less as the 
number of coats increased. 
Dipping tests.-A dipping machine was used in carrying out part of the tests, and it was 
found that a very smooth and uniform coating could be applied. The panel was completely 
immersed in the varnish and drawn out very slowly. The thickness of the coating can be 
easily regulated by using a suitable speed of withdrawal. Slightly greater moisture resistance 
is obtained by this method than by the brush method. This is due probably to the greater 
uniformity of the coating. 
Brush coating.-Practically all of the varnish was applied by brush except as already noted. 
Forced drying.- A series of panels was tested by drying in an oven at 110° F. The process 
used is as follows: Apply the varnish and allow it to set over night, place the specimens in the 
oven and dry them for four hours, remove, cool, and recoat. This method was found better 
than putting the panels in the oven immediately after coating, since there was less tendency to 
blister. 
Eflectiveness ojtke same coating applied to d~tJerent woods.-Three coats of spar varnish were 
applied to a number of specimens of about a dozen different species, and it was found that there 
was little difference in the results obtained. 
Figure 1 shows the relative effectiveness of different materials and methods of treatment 
in preventing the absorption of moisture by wood. 
"'" I 
8 ANNUAL REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS. 
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FIG. l.-The relative efiectiveness of difierent materials and methods of treatment in preventing the absorption of moisture by wood when 
exposed for 17 days to a humidity of 95 to 100 per cent. 
TREATMENT. • 
L Natural wood-no treatment. 
2. 5 coats of linseed oil plus 2 coats of wax. 
3. Impregnation treatments-
a Linseed oil (soaking). 
b Paraffin and gasoline (vacuum and pressure). 
c Beeswax (vacuum and pressure) . 
d Spar varIDSh (vacuum and pressure). 
e Cellulose varnish (vacuum and pressure). 
4. 3 coats of cellulose varnish. 
5. Filler plus 3 coats of orange shellac. 
6. Filler plus 3 coats of rubbing varnish. 
7. Filler plus 3 coats of spar varnish. 
a Poorest of 43 varnishes tested. 
b Average of 43 varnishes tested. 
c Average of 10 best varnishes tested. 
d Best varnish tested. 
8. Enamels-Filler plus-
a 2 coats enamel (red-lead pigment) plus varnish. 
b 2 coats enamel (aluminum powder) plus varnish. 
c 2 coats enamel (white-lead pigment) plus varnish. 
d 2 coats enamel (barytes pigment) plus varnish. 
e 3 coats commercial enamel (average of 11 brands). 
f 3 coats commercial enamel (best brand). 
g 2 coats laboratory mixture plus varnish (average 
of 10 best mixtures). 
h 3 coats laboratory mixture plus varnish (best labo-
ratory mixture) . 
9. Filler plus 3 coats of shellac and aluminum powder. 
10. 5 coats of bakelite plus 5 coats of varnish. 
11. Metal leaf coatings-Filler plus shellac or varnish un-
der coat plus varnish size plus aluminum leaf plus 
2 coats of varnish shellac or enamel. 
a Average of all types. 
b Best type. 
12. Sprayed with copper or aluminum and coated with 3 
coats of varnish. 
13. Electroplated with copper. 
14. Vulcanized rubber coating (1 small specimen tested). 
15. Forced drying (filler and 3 coats)-
a Average of 23 varnishes (room dried). 
b Same varnishes (dried at 110° F.). 
c Average of 5 enamels (room dried). 
d Same enamels (dried at 110° F.). 
16 Brushed versus dipped coatings-
a Filler plus 3 brushed coats (a.verage of 7 var-
nishes). 
b Filler plus 3 dipped coats (average of same var-
nishes) . 
17. Filler plus different numbers of coatings-
a 2 brushed coats. 
b 4 brushed coats. 
c 6 brushed coats. 
d 8 brushed coat . 
e 10 brushed coats. 
f 12 brushed coats. 
o 
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Moisture Resistant Finishes for Airplane Woods

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