Technical Guides

 

 

Paint Application

Application - Brush

Brushing is the most common method for applying coatings. While brushing is a somewhat slow procedure many small jobs do not warrant the use of any other application method. Brushing is especially useful for touch up work, spot priming, work in confined areas or where spraying is impossible. Less paint is wasted when applied by brush than by any other method.

Either natural or synthetic bristle brushes are suitable for use with "solvent based" coatings. Synthetic bristle brushes are preferred with "water based" coatings because natural bristles tend to swell in water.

Suggestions for good brush application:

  • Use of high quality, clean brushes of the proper shape and size will help achieve the best application.
  • Assure that all holidays or voids are eliminated, but avoid excessive brushing which may reduce film thickness and decrease protection.
  • Avoid filling the heel of the brush with material. Approximately half of the bristle length of the brush should be wet.
  • Stroke the brush at 45° angle. Light strokes at this angle ensure even flow.
  • The brush application shall be by up and down strokes, then crosswise and finally with up and down strokes lightly. This is called cross lapping and helps eliminate brush and lap marks. Fast drying materials often do not permit cross lapping which may cause paint to pile up. If it is necessary to brush apply fast dry material, it should be flowed on rapidly and generously and then left undisturbed. To go back over such a surface usually results in excessive brush drag, leaving ridges and brush marks.

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Application - Roller

Rollers are efficient tools for applying industrial coatings and are suited for broad flat surfaces. The general rule for selecting a roller cover is 'the smoother the surface the shorter the nap'. Solvent thinned coatings should be applied with either lamb's wool or synthetic covers and water reduced coatings should be applied with synthetic covers. When using rollers to apply coating systems such as epoxies and polyurethanes which contain strong solvents, be sure that the roller cover selected is constructed with glues which are resistant to these strong solvents.

Suggestions for effective roller application:

  • On large areas, material should be applied approximately on 0.75 sq.m. at a time.
  • Saturate the roller cover thoroughly with the coating. Paint should be loaded onto the cover until just before it drips.
  • The first strokes with the loaded roller should be done in a "W" pattern within approximately 0.75 sq.m. area. The "W" pattern should then be fixed in with successive strokes.
  • Work from dry areas back into wet areas. In this way a more uniform film thickness is maintained.
  • When a material is applied to warm surfaces in direct sunlight or when fast drying coatings are used, work in smaller areas to maintain a wet edge.
  • Most coatings will dry to a slight orange peel appearance when applied by rollers.

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Application - Spray

Introduction

The easiest and most rapid method for the application of protective coatings to large areas is spraying. Spray application is preferred where a smooth uniform finish is desired and speed of production is important.

Conventional air atomization is used when quality of finish is of utmost importance or where great versatility is desired. Airless spray is best for large scale operations not requiring very fine finish.

Both conventional and airless spray may be modified for increased performance or for specialized applications. One such modification includes adding heaters to fluid lines. The use of heated paint permits atomization at lower pressures, decreases or eliminates the need for thinning, cuts down an overspray rebound and provides a heavier film build with minimum waste of paint and solvents. The only drawback associated with hot spraying is that the pot life of catalyzed products sprayed by heated method is generally reduced.

Air Spray Application

A conventional air spray gun is a precision tool which uses compressed air to atomize sprayable materials. Air and paint enter the gun through separate passages and are mixed and ejected at the air nozzle to provide a controlled spray pattern. The amount of paint leaving the gun is controlled by the pressure on the fluid container, the viscosity of the paint, the size of the fluid orifice, and by the fluid needle adjustment.

Suggestions for effective air spray painting:

  • Use the lowest possible air and fluid pressure when operating a spray gun.
  • Use the proper fan width for the job.
  • Spray from the proper distance (6-10 inches).
  • Hold the gun perpendicular to the work throughout the spray stroke.
  • Move the spray gun parallel to the work surface throughout the spray stroke.
  • Move the spray gun at a speed which assures that a full wet coat is applied to the surface.
  • Overlap strokes at least 50%.

Airless Spray Application

Airless spray is a method of application which does not directly use compressed air to atomize the coating sprayed. Hydraulic pressure is used to atomize the fluid by pumping it at high pressures (500-3500 p.s.i.) through a small precision orifice in a spray nozzle. As the fluid is released at these high pressures, it is separated into small droplets resulting in a finely atomized spray.

Since air is not used to form the spray pattern, the term "airless" is used to describe this method.

Airless spray painting is cleaner and faster than conventional spraying methods. One advantage of airless spraying is that the overspray fog or rebound associated with conventional spray is greatly reduced. This makes use of equipment possible in places and applications where material formerly had to be brushed.

Another significant advantage of airless spray is the ability to apply heavy coating thickness more quickly than by any other method. Most coating materials may be sprayed in their unthinned state which also helps contribute towards the formation of heavy films and greatly reduces thinner use.

Suggestions for effective airless spray painting:

The technique employed for airless spraying is similar to the technique practiced in air spraying.

  • The ideal spraying pressure for any given tip is achieved by gradually increasing the pressure until the spray pattern appears uniform across its fan width and the atomized coating particles are of acceptable size.
  • If the coating is coarsely atomized, the pressure may be increased slightly, a smaller orifice tip used, or the coating thinned.
  • Avoid using excessively high airless spray pressures, which may cause effervescence or other finish defects.
  • Hold the gun perpendicular and move it parallel to the surface at all times in order to obtain a uniform coating of material.
  • Arcing, heeling and toeing should be avoided at all times.
  • The proper working distance with airless spray is approximately 10-15 inches.
  • When using wide angle spray tips, the gun must be moved closer (approximately 10-12 inches) to the work. Excessive spray distances increase paint fog and paint consumption.

Technique of spray application

Coating should be even and wet when spraying

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Directions of Use

Air drying epoxy and polyurethane coatings are normally two component systems, consisting of a base and a hardener. The two components have to be mixed in the ratio recommended to ensure proper and complete curing of the coatings. Improper ratio leads to problems like soft/non-dried film, poor performance, etc.

The base and hardener are to be separately mixed first to obtain a homogenous mixture. The hardener is to be added to the base slowly, with continuous mixing/agitation and not the reverse. After complete addition, mixing/agitation is to be continued to achieve a homogenous mixture. Power agitation in preferred over hand mixing.

Any addition of thinner to achieve the application viscosity should be made only after the components are thoroughly mixed and not to the base/hardener or while mixing the two components.

After thorough mixing of the product, the maturation time as indicated in the product data sheet is to be allowed before applying the product. The mixture thickens as the time progresses, and at the end of the pot life period the mixture becomes highly viscous and unusable. It is best to consume mixed paint at least one hour before the end of the pot life.

Avoid using excess thinner than the recommended volumes, since this can lead to lower dft buildup, sagging, longer curing time, etc.

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Directives for Ventilation Practice

Adequate ventilation is necessary for the safety as well the quality of the coating system. The amount and type of residual solvent in the coating can be detrimental to performance of coating, as it can affect adhesion, water resistance, mechanical and chemical properties. Very slow evaporation of trapped solvents can also develop internal stress due to shrinkage.

The ventilation must be maintained throughout the application process and also for a period after application is completed. Ventilation air should be directed to the base of the tank or compartment and should also be extracted by exhaust fans of correct balance capacity.

View Diagram

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Guideline for Application of IZS

Surface Preparation

  • Remove oil, grease and other contamination with a suitable detergent, followed by high pressure water washing. Aromatic solvents can also be used to remove the contaminants.
  • Ensure that all welds/weld seams are complete and continuous without any cracks and pinholes. Remove all weld spatters & round off all the sharp edges prior to further surface preparation. In case you find any traces of soap/alkali deposits on weld joints which are used for pressure testing, please remove the same by fresh water washing and scrubbing with stiff nylon brushes.
  • After above operation, grit blast to Sa 2 ½ to Sa 3 of Swedish specification. For severe exposure conditions, grit blasting to Sa 3 is recommended for optimum results. The surface profile after blast cleaning shall be 50 - 75 microns.
  • Substrate temperature should be at least 3°C above dew point but not above 50°C.
  • Relative humidity should be above 50%.

Note:

  • Use steel grits, aluminium oxide grits or similar sharp edged abrasives, free of foreign matters and soluble salts.
  • Steel grit with particle size of 0.2 - 1.0 mm or aluminium oxide of 0.4 - 1.8 mm should generally help to achieve the surface profile of 50 - 75 microns with 100 p.s.i. air pressure.

Application

Inorganic zinc silicate coatings should be applied by spray application only. Brush application can be used only for touch-up areas.

  • Air Spray :
  • Nozzle pressure - 43 - 57 p.s.i.
    Nozzle orifice - 1.8 - 2.2 mm
    Volume of thinner - 10 - 25 %

  • Airless Spray :
  • Nozzle pressure - 1700 - 2100 p.s.i.
    Nozzle orifice - 0.48 - 0.64 mm
    Volume of thinner - 10 - 25 %

Note:

  • Pressure pot must be fitted with an air driven agitator to maintain continuous mixing while application.
  • The fluid hose should not be more than 15 meters long.
  • Use specified thinner as given in the data sheet for thinning and cleaning.
  • Apart from correct spray technique, the amount of thinner should be adjusted in such a way that the coating applied is wet and smooth just after application.
  • The amount of thinner will depend upon site conditions such as temperature, humidity, wind flow, ventilation etc.
  • Select small nozzles for spray application of complicated and non regular structures.

Mixing of Paint

  • Stir/shake the hydrolysate part thoroughly before mixing. The powder portion (zinc dust) should be added to the hydrolysate component slowly with constant mechanical stirring. Continue stirring until the mixed paint is free from lumps.
  • Filter the mixture through a wire screen of 30-60 mesh. Keep the mixture continuously stirred during application and ensure that it is used within 4 hours.
  • Avoid part mixing of the paint.

Drying and Curing

Inorganic zinc rich primers are sensitive to environmental conditions for drying and curing. This product requires minimum relative humidity of 50% and minimum steel temperature should be 10ºC. In case skin temperature of steel is more than 40ºC, spraying shall be done by extra thinning, to avoid dry spray application.

The curing may be promoted at low humidity by spraying fresh water after 4-6 hours of application of primer and keeping the surface constantly wet until curing is complete. The complete curing can be checked by rubbing the coating with a cotton cloth soaked in thinner. If the coating remains unaffected, the curing is complete. The full hardness and curing will be obtained after weathering.

Film Thickness

The recommended dry film thickness of the primer shall be 50 - 75 microns.

Please note that high deposition of film thickness (125 microns +) can result in mud-cracking while lower dry film thickness can affect the performance of coating. In case one needs to augment the dft, apply one more coat of over-thinned primer within 24 hours.

Recoating

The most frequent problem associated when top coating is bubbling/pinholing especially with non-weathered zinc silicate coatings. To a great extent, this bubbling of finish paint can be eliminated by applying a mist coat of intermediate/topcoat as the first pass of the product, allow the bubbles to subside and then apply a full coat, as required.

Note:

In case recoating of zinc silicate with epoxy/chloro/polyurethane coatings is expected to be delayed, it is advisable to use a suitable tie coat to avoid formation of white rust. Contact the Asian Paints representative for suitable recommendation of tie coat.

The cleaning/secondary surface preparation of inorganic zinc rich primers before top coating depends on the condition of the primed surface.

Condition 1

Zinc silicate coating is intact with sporadic formation of white rust also called as zinc corrosion products.

  • Remove oil, detergent, etc., by detergent wash
  • Remove white rust by high pressure water jet washing (2000-5000 p.s.i.). If the surface is slightly contaminated, hosing down the surface with fresh water and scrubbing with stiff nylon brushes may be sufficient.
  • Ensure that the primed surface is thoroughly dry before overcoating.

Condition 2

Damaged areas, burns, weld spatters etc.

  • Remove oil, grease, dirt, etc. by detergent wash
  • Remove weld spatter
  • Spot blasting to minimum Sa 2½ followed by removal of abrasive and dust by vacuum cleaning.
  • Touch up with inorganic zinc silicate or suitable epoxy coating (Rust-O-Cap /Apcodur EHB ZP primer).

Special Instructions

  • Excessive film build up more than recommended dft may lead to mud - cracking. In such a case the coating has to be reblasted.
  • Application of second coat over the first coat should be avoided.
  • The product should not be applied on surfaces, unless, blast cleaned to minimum Sa 2½ of Swedish specification.
  • Painted structures should be kept at least 4 inches above ground.
  • Use suitable sealer/tie coat over the primer in case of project painting, where the finish paint application is generally done after erecting the structures.
  • Good ventilation is essential in confined areas.
  • Painters should wear adequate personal protective equipments e.g. air fed mask, safety glasses, etc.
  • Keep away the material from flame and sparks. Protect zinc dust against moisture.

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