Paint Application
General:
The purpose in applying a coating is to provide a prodective or decorative film to the surface being coating. The success of paint application depends on a number of parameters, including:
These parameters are discussed below.
Surface Treatment:
Surface treatment is important to the success of a coating system. So the data sheets to be an independent discuss.
Film thickness
An adequate film thickness is essential for the success of coating
system. If the film thickness is not enough, it will generally result in premature failure. However, too thickness of the film is also
dangerous. The gross overapplication of coatings can lead either to solvent entrapment and subsequent loss of adhesion. With the
majority of coatings, the limits of acceptable dry film thickness
allow for reasonable practical variation. But, the film thickness
should always be the tatget during application.
Dry Thickness Measurement
If a coating is applied to a steel substrate previously blast cleaned
with abrasive grit or shot, the measurement of its dry film
thickness is more complicated than that of a coating applied to a
smooth steel substrate. The measurement results are influenced
by the profile of the abrasive blasted surface (change from point to
point), the structure of measuring instruments and dry film
thickness. Sunrui recommend that all measuring instruments are
calibrated on smooth steel in accordance with GB/T 13452.2-2008.
When thin films are being applied care should be taken to consider
the surface profile whereby some of the coating is being used to fill
in the profile.For primier and recoating of less than 25 microns,
measurement over the blasted surfaces is not meaningful. Consult
Sunrui sale department for recommended measurement.
Method of Application
The methods of application described in the data sheets include
brush, roller, conventional air spray, conventional spray with pressure pot and airless spray.All methods are briefly discussed below.
Brush
Brush application should always be undertaken using good quality natural fibre or synthetic brushes of the appropriate size compatible with the coating being applied.Hower this application technique is relatively slow, but is generally used for coating small areas with decorative paints and for surface tolerant primers, where good penetration of rusty steel substrate is required. It is particularly suitable for the application of stripe coats and for coating complex areas where the use of spay methods would lead to considerable losses due to overspray and associated dry spray problems. Hower most high build coatings are designed for application by airless spray, and high film build generally not be by brush application. In general, twice as many coats will have to be applied by brush to achieve a similar build when compared to airless spray. Brush application requires considerable care when applying ono-convertible coatings over one another,such as chlorinated rubber over chlorinated rubber, or vinyl on top of vinyl coat. Even a a mild degree of the brushing-out normally given to topcoats will cause pick-up of the previous coat and result in a very poor finish. Light stroke should be used in these circumstances, covering a particular area with one or two brush strokes, and on no account working the bristles into the previous coat.
Roller
Roller is faster than brush on large, and it can be used for the application of most decorative coatings. However , film thickness is not easily to be controlled. As with brush application, high build will generally not be attained. It must be according to the type of paint and degree of roughness of the surface to choose the correct roller pile lengthand material. Generally, phenolic core rollers fitted with a smooth to mediumpile roller cover should be used. The roller cover should be prewashed to remove any loose fibres prior to use.
Conventional Air Spray
This is a widely accepted, rapid method of coating application in which coating is atomized by a low pressure air stream. Conventional air spray is relatively simple and inexpensive, but it is essential to use the correct combination of air volume, air pressure and fluid flow to give good paint film free from defects.
If conventional spray application is not controlled correctly, large losses of paint can result from overspray and rebound from the surface in addition to problems such as poor flow, sagging and pinholing. The major disadvantage of conventional air spray is that high build coatings can generally not be applied by this method as most paints have to be thinned to a suitable viscosity for satisfactory atomization, and so lose their high build properties.
Conventional Spray with Pressure Pot
Pressure feed tanks or “pressure pots” are commonly used in association with low pressure air stream spray guns, to provide a means of delivering paint at a regulated pressure from a tank, through a fluid hose to a spray gun. Several manufacturers produce suitable equipment (Devilbiss, Binks) which operate in the following manner. A length of air hose from the compressed air supply is connected to an air pressure regulator on the tank lid. Some air bleeds through the regulator at an adjusted pressure into the tank but most of the air passes the regulator and reaches the spray gun through a second length of air hose to atomise the paint
as it is sprayed. The air which has entered the tank forces paint from it to the gun through a length of fluid hose. Paint in the tank
can be prevented from settling by means of an agitator driven by
hand or by a compressed air motor. Under the circumstances of
use a lot of coating, spray with pressure pot is recommended.
Since this method instead of the spray gun connected with suction
tank and gravity tank, it can reduces waste time in constant
refilling, and also enables the gun to be turned to any angle to coat
objects effectively without spilling paint.Pressure feed tanks up to
20 litres capacity can be used and allow ease of movement around
the workplace.
Airless Spray
Unlike air spray, airless spray is not mixed air and coating.
Atomisation is achieved by forcing the paint through specially designed tips, by hydraulic pressure.
The required hydraulic pressure is usually generated by an air powered pump having a high ratio of fluid pressure to air input pressure. Pumps with ratios between 20:1 and 60:1(or greater) are available, perhaps the most common being around 45:1.
The chief advantages of airless spray are:
The tips (the paint is forced to achieve atomization) are precisely constructed from tungstencarbide. The atomized fan is produced by a slot ground on the face of the orifice. Various orifice sizes together with different slot angles are available. The choice of tip is governed by the fluid pressure required to give atomization coupled with the orifice size needed to give the correct fluid delivery rate.The fluid delivery rate controls the film thickness applied per pass of the spray gun
Different slot angles produce spray fans of different widths. The selection of a particular fan width depends on the shape and size of the structure to be painted. Choice of fan width is also related to orifice size for the same orifice size, the paint applied per unit area will be less the wider the spay fan.
Normally, airless spray equipment orerates at fluid line pressures up to 352kg/cm2, and should always be used in accordance with the operating instructions and safety precautions of manufacturer. Generally, tips with an orifice size 0.23-0.33mm are suitable for coating to be applied at approximately 50μm wet film thickness. Tip sizes from 0.33-0.48mm for wet films of 100-200μm, and 0.48-0.79mm for 200μm and above. Heavy duty anticorrosive coating may need tips with orifice as large as 1.02-1.52mm.
There are several designs of tips available, the choice of which depends upon the finish required, the ease of application and ease of clearing blockages from the tips. With some products, the decorative effect achieved with airless spray is not as good as can be achieved by conventional spray. However, airless spray application is now widely accepted as a convenient method of applying marine coatings.
Conditions
In the paint application, it is important to consider the status of substrate, thesurface temperature and atmospheric conditions. Paint should be applied with good atmospheric conditions.
Under the following conditions, painting should not be taken:
At night, steel temperatures fall and then rise again during the day. However, compared with the atmospheric conditions, there is always a lag in movement of steel temperature. So condensation on the steel surface is possible. Condensation will occur if the steel temperature is below the dew point of the atmosphere.
Borderline Conditions
Bad weather is a familiar problem to those using coatings. Relative humidity itself rarely creates a problem. Most paints will tolerate high humidityes,but humidity should not be permited to lead to condensation on the surface being coatinged. In order to determine whether or not a surface is wet, the steel temperature should be measured using a surface temperature thermometer and the dew point calculated after measurement of humidity with a hygrometer. Paint appkication should not take place when steel temperature is less than 3℃ above the dew point.
Paint should not be applied when surfaces are affected by rain or ice.Some two component paints(such as certain epoxy coatings) should not be applied at low temperatures as curing my be retatded.
Extreme Conditions
Generally, extreme conditions refer to ambient temperatures below 5℃ or above 40℃. Below 5℃ the curing of coatings such as traditional two component epoxies, slows down dramatically and for some paints curing stops altogether. Other marine coatings are not so severely affected. Chlorinated rubbers and vinyls are quite suitable for use at temperatures below 0 ℃ provided that the surface is clean and free from ice or frost.Another extreme of 40℃ and above, the drying and curing of paints is rather rapid and care should be taken to avoid dry spray. This is caused by the too rapid loss of solvent from paint droplets between the spray and the surface.It can be avoided by:
In conditions of high temperature, techniques must be adopted to prevent defects such as void, pinholes, bubbles, and poor coverage due to the over rapid evaporation of solvent. However, provided that good standards of workmanship are maintained, it is normally possible to satisfactorily apply most Sunrui coating on to steel substrates up to 65℃.
