Paint Application

Paint Application

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: Surface Treatment film thickness Method of coating Condition during application...
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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:

  • Surface Treatment
  • film thickness
  • Method of coating
  • Condition during application

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 medium

pile 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:

  1. High build coatings can be applied without thinning.
  2. Very rapid application is possible, giving an economic advantage.
  3. Compared to conventional spray, overspray and bounce back are reduced, leading to reduced losses of material and less dust and fume hazards.

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:

 

  • When the air temperature is lower than drying and curing limit of the coating.

 

  • Fog or frost, or when rain or snow is coming.

 

  • When the surface to be coated is wet with condensation or when condensation can occur during the initial drying period of the coating.

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:

 

  1. Keeping the spray gun at the minimum suitable distance from the work piede, spraying consistently at 90° to the surface being painted.

 

  1. Adding thinners, if necessary, up to a maximum of 5% by volume.

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.

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