PAINTS FOR DIPPING.
It need hardly be said that the paint employed for this purpose must be
of a special character. There are several firms of manufacturers who
make a speciality of this class of paints and supply them either ready
for use or in paste form, requiring only the addition of white spirit
or other suitable thinner to bring them to the right consistency. The
following may be considered as the necessary qualities of a paint
suitable for this purpose:--
1. It must dry hard in not less than six hours.
2. The pigment must be of such a specific gravity that when mixed with
a suitable vehicle or thinner to form a paint the pigment will not
quickly deposit at the bottom of the tank or settle out.
3. The consistency of the paint must be so arranged that only a minimum
quantity will run off, while it must not be so thick as to give rise to
tears or runs.
White lead which has a specific gravity of about 6.750 is for the
reason mentioned usually considered too heavy, and in the case of a
white or grey paint, zinc oxide which has a specific gravity of 5.470
is used in preference. The following colours may also be successfully
employed, because of their low specific gravity, viz., Oxford ochre (s.
g. 2.822), Venetian red (s. g. 3.560), Indian red (4.732), golden ochre
(3.107), Italian raw sienna (3.081), burnt sienna (3.477), Turkey umber
(3.496), Prussian blue (1.956), bone black (2.319).
It will be observed that the earth colours, such as ochre, sienna,
umber, etc., are all light pigments, and as they are also recognised
as being the most durable, they are eminently well suited for use in
many positions, although the colour is not always very agreeable.
Precipitated barytes (s. g. 4.144) can be used up to, say, a proportion
of 10 per cent.--not more; red lead (s. g. 8.681) cannot be used for
this purpose, as it is far too heavy, and the same applies to English
vermilion (s. g. 7.726). If, however, a bright colour is required, it
may be obtained by dipping in Venetian red or Indian red and giving a
second coat of crimson lake (s. g. 1.898), but a coat of varnish should
be sprayed upon this for protective purposes.
In considering this subject, it is well to remember that much will
depend upon the thickness of the vehicle used, as, clearly, a fairly
heavy pigment will settle out in a thin vehicle much quicker than it
would in a comparatively thick one. The successful paint, therefore, is
one in which both considerations are taken into account.
Very rarely indeed does it pay manufacturers to attempt to grind or
prepare paints themselves, and much greater satisfaction will be
obtained by getting supplies from a reputable firm who have made a
special study of the subject.
Sometimes a white paint is required for dipping, and in that case
30 lbs. of sublimed white lead, 10 lbs. of zinc oxide and 6 lbs. of
gilders' whiting, with 4 lbs. of asbestine pulp, mixed with 9 lbs. of
raw linseed oil, will be found to give good results.
A paint which dries with a gloss is often desired for the finish of
many articles, and can be readily obtained by first priming, then
giving a finishing coat of paint on it which has been mixed with
sufficient varnish to produce the desired gloss. In some cases three
coats may be given, namely, the first or priming coat, the second,
which should be flat, or semi-flat, and the third a coat of varnish
paint, which will dry with a gloss. It should be pointed out, however,
that the finish obtained by these means is not a little inferior to
that which may be produced by using over the priming one or more coats
of flat paint of the desired colour and finishing with a coat of
suitable varnish, which may be applied either by dipping or spraying,
according to circumstances. A point here worthy of mention is one which
every house painter is or should be well acquainted with, and that is
that the coats of paint that are superimposed should be alternatively
flat, _i.e._, without gloss, and glossy, in order that each may adhere
closely to the other. If the article to be painted is wood or any other
material which is of an absorbent character, a comparatively large
amount of turpentine and oil must be mixed with it in order to allow
for suction. This will dry with a semi-flat finish, and a glossy coat
or one having more oil in its composition, may be applied over it. If
a further coat is required it should be flat or nearly so, and in that
case a finishing coat of varnish will probably be required. The paints
for these purposes may be purchased ready made or ready for thinning
down, from firms who have made a special study of the requirements.
The question sometimes arises as to whether paint dipping or spraying
can be used advantageously when in the finish two or more colours are
to be used. As a rule the difficulties can be overcome by dipping first
or even the second coat, spraying on the third or finishing coat, using
specially prepared masks or shields over those parts which are not to
be painted with the particular colour in use.
The following useful information is taken from "White Paints and
Painting Materials," by W. G. Scott, who was for sixteen years
connected as paint expert with the Milwaukee Harvester Company and the
J. I. Case Threshing Machinery Co. Mr. Scott therefore speaks from a
wide experience. He says:--
A certain amount of oil must be present in dipping paints to act as
a binder, and it is advisable to add a small quantity of varnish to
hold the paint together. The desired features in a good dipping paint
are: freedom of flow and proper drip; sufficient binder to prevent
chalkiness and produce a firm coat; covering capacity and an even
distribution of the paint.
Non-absorbent surfaces like metal and hard wood require less oil than
the absorbent soft woods.
In the former case there is little or no penetration of the liquid
portion of the paint, but with pine, bass wood, white wood, poplar,
etc., nearly all of the liquid is absorbed or taken up by the
wood, consequently with benzine only as a thinner there will not
be sufficient binder present to hold the pigment when the thinner
evaporates.
Whereas 5 lbs. of paste pigment or colour ground in oil and thinned
with a gallon of benzine would produce a suitable primer on iron or
other non-absorbent material, it would not answer for soft wood.
Dipping paints, as a rule, contain from 4 to 10 lbs. of paste per
gallon of thinner, the primer containing less paste than the second
coat paints.
The composition of the paste colour has much to do with the dipping
paint; for instance, 5 lbs. of paste white lead to the gallon of
thinner will cover and work better than a mixture consisting of half
white lead and the other half made up of transparent pigments like
china clay, barytes, etc., nevertheless a small amount of inert
material is generally understood to be an improvement in the way of
durability.
Asbestine, whiting, silica and china clay are the inert materials most
often used in paste goods for dipping purposes.
Asbestine probably helps to hold the pigments in suspension better
than any of the others and answers nicely for dipping paints, but, as
previously mentioned, does not allow the paint to level out when used
with a brush.
China clay, on account of its low specific gravity, is much favoured as
a suspension agent, but materially lessens the opacity of the paint.
A small amount of whiting is a good addition to a dipping paint, as
it carries down much of the dirt and heavy particles usually produced
during the process of dipping. Silica gives the paint "tooth," and by
some authorities is considered an actual necessity in primers which are
intended to be sand-papered.
White lead and zinc oxide are the two ideal white pigments considered
from a dipping standpoint, and it will be found that the majority of
dipping paste paints on the market contain a notable quantity of zinc
oxide, either straight or in the form of zinc lead.
Zinc oxide is generally the predominating pigment in the white and
tinted paste paints, and is usually associated with white lead (basic
carbonate), zinc lead, and sublimed white lead, mixed with more or less
inert material according to the ideas of the manufacturer or in order
to cheapen the product.
The admixture of inert material with the coloured pigments requires
some knowledge of the composition of the coloured pigment; for
instance, lampblack might be safely mixed with asbestine, whiting,
silica, barytes, etc., without detriment, but yellow ochre containing,
naturally, considerable clay and silica would hardly permit of any
great addition of china clay or silica.
The strong iron oxides, chrome greens and similar tinting colours will
stand a large quantity of inert material when the paint is to be used
as a body colour, but for varnish colours it is customary to use the
chemically pure colours and less paste per gallon of thinner.
Paste paint for varnish colours is frequently ground in oil, in japan,
or in a mixture of the two, but far better results are obtained by
grinding the dry pigment in varnish thinned to a grinding consistency
with a little turpentine. A varnish with a viscosity of 20 (water = 1)
which gives a stiff mix with, say, 5 lbs. of pigment will, when thinned
to a viscosity of that of raw oil, _i.e._, a viscosity of 4, take about
20 lbs. of pigment.
No set rule can be given for the kind and amount of thinners to
be used in dipping paints for various purposes, but the following
proportions[1] will furnish a key to the general mixtures used:--
[Footnote 1: The figures given relate to an American gallon, which
contains 231 cubic inches, while an English imperial gallon contains
277¼ cubic inches. For practical purposes the latter may be
considered one-fifth greater than the former.]
PRIMERS FOR METAL.
4 to 5 lbs. of paste thinned with 7/8 to 31/32 gallon of benzine or
turpentine and 1/8 to 1/32 gallon of mixing varnish.
PRIMERS FOR HARD WOOD.
4 to 5 lbs. of paste thinned with 15/16 gallon benzine or turpentine,
3/64 gallon raw oil, 1/64 gallon mixing varnish.
PRIMERS FOR SOFT WOOD.
4 to 7 lbs. paste thinned with ½ to ¾ gallon benzine or turpentine,
15/32 to 15/64 gallon raw oil, 1/32 to 1/64 gallon varnish.
In some cases, with very soft, porous woods, it may be necessary to add
more raw oil and some japan or liquid drier, but too much drier must
not be added, as it will shorten the "flow."
SECOND COAT DIPPING PAINTS.
5 to 10 lbs. of paste thinned entirely with benzine or turpentine, or
with 7/8 gallon of solvent and variable proportions of oil and varnish
according to the surface desired.
Varnish is advocated in all of these mixtures, as it helps to hold the
solids and liquids together and prevent separation; it also induces
toughness.
It is essential that a varnish be used which will mix perfectly with
oil and benzine at a temperature of 60° F.
Varnish colours are best thinned with turpentine, but owing to the high
price of turpentine the large factories insist on using benzine or some
of the turpentine substitutes now on the market.
The fact that some of the turpentine substitutes work better and give a
better "flow" with the paint and varnish, is due to the fact that most
of them contain a heavy distillate of petroleum similar to kerosene.
Kerosene oil has the double property of thinning and imparting "flow"
to either paint or varnish, but retards the drying, hence too much must
not be used.
Damar varnish, which will not stand thinning with benzine without
separation of the gum or becoming cloudy, will permit of dilution with
kerosene to quite an extent.
So far as durability is concerned, kerosene imparts more durability and
is more waterproof than any of the other thinners. Combined with rosin
and manganese oxide, it may be made to dry like raw linseed oil, but,
of course, does not possess the same properties.
In dipping paints, as in all other kinds of paints, good judgment must
be used in the mixing, and it is absolutely essential that paints for
soft wood, or other absorbent material, contain enough oil, or binder,
to hold the pigment.
WHITE PASTE PRIMER. EXTRA FINE.
300 lbs. white lead (carbonate).
150 " zinc oxide.
50 " floated silica.
Grind in
10 gal. raw linseed oil.
Product = 575 lbs.
This paste is thinned with turpentine or benzine for dipping purposes
in the following manner:--
WHITE DIP FOR METAL.
100 lbs. of white paste.
14 gal. turpentine or benzine.
½ " pale mixing varnish.
¼ " white liquid drier.
WHITE DIP FOR HARD WOOD.
100 lbs. white paste.
13 gal. turpentine or benzine.
2 " raw linseed oil.
¼ " pale mixing varnish.
½ " white liquid drier.
WHITE DIP FOR SOFT WOOD.
100 lbs. white paste.
12 gal. turpentine or benzine.
6 " raw linseed oil.
¾ " white liquid drier.
The pale mixing varnish and the white liquid drier must not liver with
lead or zinc, otherwise, the paint in the tank will thicken continually.
The above formula produces one of the best paints known, and it is
certainly not a cheap paint.
WHITE SPIRIT.
As already mentioned, many firms who use the paint dipping process
buy their paint in paste form with the requisite amount of driers
added, and thin it themselves with white spirit, which is a petroleum
product specially prepared for the purpose. Turpentine is, of course,
too expensive, and the spirit costs, under normal conditions, little
more than one-third of the price, although exact figures cannot be
given owing to the fluctuation in the price of these materials. Some
manufacturers use a white spirit, to which has been added from 25 to 30
per cent. of genuine American turpentine. This, of course, renders the
mixture somewhat expensive, and possesses no advantage excepting that
it gives the characteristic smell of turpentine; but this, it appears,
is not important in a factory, and the pure white spirit is quite
suitable in itself.
According to Mr. S. Roy Illingworth, A.R.C.Sc., A.I.C., B.Sc., Lond.,
expert to the Gas Lighting and Improvement Co., Ltd., turpentine
substitutes were first placed on the market about 30 years ago, and
consisted at that time of a mixture of turpentine and kerosene oil.
Investigations led to the production of white spirit, drying as quickly
as turpentine, or even quicker if desired. Several grades of this
material are made, varying in flash-point from 80° up to 150°. The
flash-point of genuine turpentine, it may be mentioned in passing, is
90° to 91°, and the 90° flash-point spirit is the one usually employed
for painting purposes, although the 80° flash-point would answer
equally well for dipping. The best grade is practically free from smell
and is suitable for admixture in all paints, excepting those having a
bitumastic or asphaltic base, or paints in which very common driers
containing cheap rosin are used. A rough-and-ready test as to the
quality of white spirit is to dip a piece of blotting-paper into it
and hang it up to dry. In the course of an hour-and-a-half it should
have wholly disappeared, leaving no smell or stain behind it. If there
should be a stain the sample should be viewed with suspicion. Another
test of the same kind is to drop a little spirit on a piece of white
writing paper and to leave it for three-quarters of an hour, when it
should have disappeared without leaving a stain. In addition to these
tests it might be advisable to mix a little spirit with paint and to
try it experimentally.
In the evaporation of turpentine it is usually admitted that a small
proportion, say, from ·25 to ·50, is left behind owing to oxidation.
In the case of best white spirit, however, very little residue will
be left behind, certainly not sufficient quantity to interfere in any
way with the drying. The use of the spirit has proved so successful
that practically nothing else is now employed in paint dipping. It is,
however, important, of course, that the best quality white spirit be
used, but as this is so much less in cost than turpentine, there is no
temptation to use an inferior quality.
THE QUANTITY OF PAINT REQUIRED FOR DIPPING AND SPRAYING.
Speaking generally, rather less paint will be required when it is
applied by dipping, or spraying, than is necessary when it is applied
with a brush. But there are many exceptions, particularly in spraying,
where it may be necessary to give a coat considerably thicker than
usual.
The following table will be found useful as a guide, but it should be
remembered that the actual space covered will vary considerably with
different grades of paint, so that the table should only be taken as
approximately correct. In cases of doubt, careful calculations should
be made after a given quantity of paint, varnish, or lacquer has been
applied to a surface of known area. This will serve as a useful guide
in ordering in the future. It need hardly be said that the quantity of
paint necessary will vary largely with the surface to which the paint
is applied. For example, an absorbent surface, such as unprimed wood,
will require much more paint than metal work, which absorbs little or
none.
SPREADING CAPACITY OF PAINTS.
Per Gallon Per Gallon
on Wood. on Metal.
Priming Coats 60 to 70 90 to 100
Mixed Paint 2nd coat 85 to 90 100 to 120
Varnish (on paint) 95 to 110 120 to 130
Lacquer (on paint) 100 to 115 130 to 150
Enamel (on paint) 75 to 80 85 to 95