CORROSION AND PAINT 481
suiiace decorative work. These paints are supplied ground in oil so
that, on board ship, it is only necessary to add linseed oil and stir well
to reduce it to the desired consistency; add a small quantity of driers.
It works out as a rule that 1 quart of oil reduces 6 Ibs. of paint to a
suitable consistency having an average covering power of 50 square
yards. The pigment settles to the bottom of the pot and so must be
stirred with a stick at frequent intervals during painting operations,
When the job is finished any residue of paint should be poured back
into the container and the pot wiped out clean to be ready for next
time, otherwise, skins quickly form on the inside of the pot if paint is
left in it.
Brushes should be rubbed out as dry as possible and put in water
overnight, but if they are not to be used again for some time they
should be washed out in hot water and soap. Varnish brushes should
be put in linseed oil to keep them soft. All brushes when stored away
should be hung up or laid flat and never left standing on their bristles.
Bottom Compositions.—Underwater paints are applied in dry dock.
They are classed as anti-corrosive, anti-fouling and boot-topping.
They are expensive paints, specially manufactured with heavy pigments
which settle rapidly and have to be kept stirred continuously during
application, and a quickly evaporating vehicle which causes the paint
to dry almost as quickly as it is put on. The job has to be done quickly.
Anti-corrosive is first applied to act as a foundation and as an
insulator to prevent the destruction of the steel. The plating must be
first freed from grease and scum as the paint will not adhere to a greasy
surface and will soon flake oS.
Anti-fouling is coated over the anti-corrosive, its purpose being to
retard marine growth, barnacles and grass mostly. Anti-fouling com-
position consists largely of oxide of mercury, and if it comes into contact
with the bare steel it will set up corrosion, hence the need'of an anti-
corrosive as an insulator. Anti-fouling coating is usually carried up to
the light load line only and a cheaper paint, called, boot-topping, applied
between the light and load water marks when the ship is afloat.
The cellular double bottoms, inside of tanks, are usually coated
with a layer of cement deep enough to cover the rivet heads
(mixture—two parts sand to one of cement), then cement-washed
occasionally. See also page 613.
Wetted Surface.—The area of the underwater form of a ship is
given by the equation:—WS=L (l*7d+CbxB) where