Application of paints

Dr. Dmitri Kopeliovich

Paint is applied onto a substrate surface which has been previously treated by the
methods of Enhancement of adhesion (surface preparation): Cleaning, Surface
roughening, Primers, Adhesion promoters, Plasma treatment.

The following techniques are used for application of paints:

 Brush
 Roller
 Air spray
 Airless spray
 Electrostatic spray
 Rotating disc or bell
Brush
Brush application is used for decorative coating of relatively small and complex areas.
Brush painting is a slow application method but it provides good penetration of the paint.
Brush application prevents paint losses associated with the spray application method.

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Roller
Roller technique is applied for coating large flat surfaces.
The method is faster than brush application.
In contrast to the brush method roller painting is not suitable for applying high film
build.

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Air spray
In the air spray application method the paint in form of atomized droplets is delivered by
a stream of compressed air.
The paint is pumped from the tank into the gun where it is atomized (broken into small
droplets) by the flowing compressed air.
The typical air pressure in the air spray is 40-50 psi (0.28-0.34 MPa). Spray application
is a simple and inexpensive method which is capable to coat objects of various shapes
and dimensions.
High quality coatings are achieved if the paint composition (the content of Solvents)
provides the required levels of viscosity and the drying rate.
The disadvantage of the air spray application is unavoidable overspray: losses of the
paint and expensive organic solvents and thinners.
Besides the costly losses the method requires additional investment in safety equipment
in order to protect the workers exposed to the hazardous solvent vapors.

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Airless spray
In the airless spray (in contrast to the air spray) the paint is not mixed with air.
The liquid paint is delivered to the gun by a hydraulic pump producing a high pressure
300-7000 psi (2-48 MPa).

The pressurized paint is atomized in the gun nozzle. The nozzle is commonly made of
tungsten carbide. It has an orifice, diameter of which (together with the paint viscosity
and the pump pressure) determines the delivery rate and controls the coating thickness.
Using high pressure pumps and preheated paints allows to apply high viscosity paints
 having a decreased amount of solvents and thinners (e.g. High-solids paints). Lower
solvent content in the paints for airless application results in a reduced overspray.

A combination of the airless and the airless application methods is used in the air
assisted airless spray application technique.
The paint is supplied to the gun and atomized like in the airless method. However the
paint is additionally atomized by low pressure compressed air which also controls the fan
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Electrostatic spray
Electrostatic spray application is based on the electrostatic attraction of electrically
charged atomized particles (either dry powder or liquid paint droplets) to the grounded
(having earth potential) workpiece.
The charged particles coming out from the gun hit the whole surface of the object
including the back surface therefore the electrostatic spray technique is mainly used for
inside coating of tubular objects where the back surface painting (wraparound effect) is
negligible.

Due to electrostatic effects the coating is thicker on the surfaces of protruding corners
and thinner on inside corners and holes.
Electrostatic spray is characterized by very low overspray. The method utilizes at least
95% of the paint
Rotating disc or bell
In the rotating disc/bell application technique the liquid paint is pumped to the center of
the disc/bell rotating at high speed (at least 10,000 RPM).
The centrifugal force drives the liquid to the edge where it is atomized and spun off.
The rotating disc commonly has an electrical potential which provides an electrical
charge to the paint.
The paint particles propelled by the centrifugal force are attracted by the grounded
workpieces located around the rotating disc.
Rotating disc or bell method is used for automatic application of high viscosity
paints(e.g. High-solids paints) in conveyor lines.

Applying paint
Using a roller
Using a brush
Cutting in
Care of brushes
Paint can be applied by brush, roller or spray. Spray may seem a faster way of getting the
job done, but it requires experience to get an even and unblemished surfaced and is best
left to professional applicators. Spraying also requires care with masking and protecting
nearby surfaces.
There are some general rules:
 Don’t paint when temperatures are below 10°C (the paint may take too long to dry) or
above 25°C (the paint may dry too quickly or dry on the brush or roller).
 Try not to paint in full sun. This is especially true for roofs, where painting on a cloudy
day works best.
 Don’t paint outside if rain is forecast.
 Don’t paint with oil-based paint beyond the middle of the afternoon, as the paint may
not be dry enough by the time overnight moisture falls.
 When using oil-based paints on windows and doors, paint early in the day to allow
plenty of time for the paint to dry before they need to be shut.
 Don’t thin the paint unless the paint label specifically directs this.
 Allow one coat to dry properly before applying the next – see the instructions on the
paint container.
 Very lightly sand between coats with solvent-borne paint to remove any uneven
undercoat application or dust particles.
 With exterior painting, ensure primer is overcoated within four weeks of application.
 When painting walls, start at the top and work down. Outside, paint soffits first; inside,
paint ceilings first.
USING A ROLLER
Rollers are best for applying paint rapidly to large flat surfaces such as interior
plasterboard-lined walls and ceilings.
Select the for the type of surface being painted and the type of finish you prefer.
Short pile rollers hold a smaller amount of paint, give a smoother finish, and are best used
on smooth surfaces. Medium and long pile rollers hold larger amounts of paint and are
suited for semi-rough and rough surfaces. The longer the pile, the more noticeable will be
the stippling or 'orange peel' effect in the finish. This can mask small defects in the
surface.
Foam plastic rollers are suitable for solvent-borne (oil-based) paints on smooth surfaces.
To successfully apply roller paint:
 Paint areas that the roller can’t reach (such as edges) with a brush first, then overlap the
painted area with a roller while there is still a wet edge.
 Load the roller by immersing it in paint (to one third of its diameter), then run it
backwards and forwards across the corrugated area of the tray to evenly wet it.
 Roll the paint onto the wall so the roller is almost dry as it bridges the gap to the wet
edge (the last painted area) of the section previously painted.
 Always work in the same direction (up and down on vertical surfaces) and work away
from the main source of light.
 Roll over the area painted with a near-dry roller to ‘lay-off’ the surface to give an even
finish.
 Do not force the roller onto the surface being painted or over-roll the surface (this will
cause patchiness).
 Maintain a wet edge of paint to roll into wherever possible (although when painting
large areas it may not always be possible to maintain one).
Before first using a roller, clean it to remove dust and other particles that may affect the
paint finish.
When a roller is being used on consecutive days, it can be covered tightly in cling film
and stored in a cool place overnight.
After finishing painting, run the roller over sheets of newspaper to remove excess paint.
Clean with an appropriate solvent (see paint container label), wash in warm soapy water
and rinse thoroughly in clean water.
Hang rollers to dry in a way that the pile is not flattened.
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USING A BRUSH
The correct way to brush on paint is to:
 use a brush that is suitable for the type of paint being used and the material/surface
being painted
 when painting large areas use the largest brush that you can comfortably handle
 for intricate areas – such as fretwork on an old villa veranda – use a brush small enough
to get into the crevices
 load the brush with paint, then remove excess paint by running the brush across the lip
of the container. This reduces the risk of paint dripping from the brush
 use sufficiently firm pressure for the paint to properly coat the surface
 brush the paint in 2 directions, in general in the direction the light is shining
 always brush towards the wet edge. A better finish will be achieved, especially with
acrylic paints, when the paint is brushed into an edge that is still wet
 when finishing to a corner, brush out from it to meet the new paint
 ensure the paint is evenly applied
 ‘lay off’ the wetted area (laying off is applying the finishing strokes) with light brush
strokes in the direction of the grain for timber and towards the main source of light for
flat surfaces.
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CUTTING IN
Cutting in refers to applying a paint edge or line where two colours or finishes adjoin,
generally without using masking tape. This may be in a corner or change of direction of a
surface such as a ceiling/wall intersection or a wall/architrave intersection and when
painting adjacent to the glass of timber frame windows.
A cutting-in brush, where the bristles are tapered, facilitates cutting in.
 To cut in:
 use a small brush, typically no more than 50–60 mm wide, for cutting in work
 dab the brush into the paint and drag it ‘dry’ against the edge of the paint container – do
not over-load the brush
 using the sharp edge of the bristle wedge, drag the brush in a steady line along the
required paint edge
 continue until most of the paint has been left on the surface
 repeat the procedure until the edge painting is complete.
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CARE OF BRUSHES
Cleaning and looking after brushes will mean a better painting result.
 If painting in hot weather, clean the brush during the day to prevent it becoming clogged
with dried paint.
 When a brush is being used on consecutive days, it can be covered tightly in cling film
and stored in a cool place overnight.
 After the painting job is finished, clean the brush thoroughly with the right solvent (see
paint container label), wash in warm soapy water and rinse thoroughly in clean water.
 Keep brushes in dry storage, hung by the handle or resting flat.
 Never store brushes for more than just a day by standing on their bristles in a container
of water or solvent.