Khanna A.S. (Ed.) High-Performance Organic Coatings: Selection, application and evaluation

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Surface preparation for organic paint coatings 35

surface just before it enters the coating booth, while chlorides are measured

by adding silver nitrate solution in the water solution obtained by wetting

ence of chlorides. There are several commercial methods such as the test

kit from Chlor-rid. [4].

2.5 Sponge jetting

Sponge jet is a low-dust abrasive blasting technique for surface preparation.

It suppresses up to 99.9% of what would normally become airborne dust

[5]. Sponge Media abrasives are manufactured with a tough, porous ure-

thane sponge material, which controls or suppresses dust. Sponge Media

abrasives rebound, the porous urethane creates suction, entrapping dust.

Compressed air is used to propel particles against the surface to be cleaned.

After use, the sponge and debris are collected and loaded into a vibrating

screen, and the reusable portion of the sponge is returned for additional

blast cleaning.

The specialized equipment required for sponge jetting includes an air

compressor, blast hoses and special blast nozzle, specialized blast pot, abra-

sive collection equipment (e.g., vacuums), and vibrating screen abrasive

reclaiming equipment. Screens or tarps to isolate the work area along the

to the separation system [1].

There are several advantages of sponge blasting, with a few limitations

tion, with low paint debris with more chances of using recycled sponge

abrasive, and surface cleanliness of the same or better quality than

for abrasive blasting, overcome the limitations. Low waste generation

compared to abrasive blast cleaning, with increased worker and public

safety by minimizing adverse effects to the environment, are additional

advantages.

2.6 Hydrojetting (water blasting)

Hydroblasting is a technique for cleaning surfaces that relies entirely

on the energy of water striking a surface to achieve its cleaning effect.

Abrasives are not used in hydroblasting systems. Consequently the prob-

lems caused by dust pollution and by the disposal of spent abrasives are

eliminated.

It may be mentioned here that not all water-blasting techniques qualify

as hydroblasting. As a matter of fact, depending upon the pressure of the

a small blasted area. White precipitates of silver chlorides confirm the pres-

they cut into the coating and substrate profiling. As the Sponge Media

ground are typically sufficient to capture the abrasive and debris for return

of slightly higher cost and low efficiency. The advantages of dust-free opera-

particles flatten as they strike the surface, then expose the abrasive where

36 High-performance organic coatings

ing. Water pressure up to 5000 psi usually does not remove any scale but

helps in removing dust, soluble matter and other suspended impurities.

Raising the water pressure to 10 000 psi just helps to remove loose contami-

nants, but adherent corrosion layers are not removed. However, such a

pressure can cut steel plates.

Hydrojetting is usually described from a pressure of 10 000 psi to

25 000 psi. A pressure of 25 000 psi can be used to remove scales of grade

A and B, but is not effective on scales of grade C and D. These days pres-

sures of 25 000 psi to 45 000 psi are used to clean the thick corroded layers

formed on the surface. The latter is called Ultra High Pressure hydrojetting.

One precaution is required while using hydrojetting as one is handling

water at very high pressure, thus the distance of the object must not be very

nozzle must be about 50 mm from the substrate.

The main advantages of hydrojetting are:

• It is a quicker process, several times faster than abrasive blast cleaning.

• For thick scales, only blasting needs to be carried out, without having to

done in abrasive blasting, followed by the abrasive blasting itself.

• Because water is a solvent, many salts, dust and other impurities are

removed in a single step, thus avoiding any pre-cleaning as is required

in abrasive blast cleaning.

• No requirement for costly abrasives, nor any need of collection after the

blasting process.

• Simple process of a high-pressure pump and a water tank connected to

special nozzles which give a very high-pressure jet.

There are, however, several disadvantages of this process. A few impor-

tant ones are as follows:

• A large amount of good quality water is required and hence the method

is most suitable where water is available in plenty. Use on offshore

structures and in remote areas is limited.

•

ing with abrasives. There is an alternative of using abrasive water blast-

ing or slurry blasting, discussed in the next section.

•

serious problem which has been solved in recent years by one of the

following methods:

water, the blasting method can be classified as water cleaning or hydroblast-

It can give efficient cleaning with water consumption of 8 l/min.

far. It is expected that in order to get good efficiency from the hydrojet, the

perform different steps of first removing the scales by power tools as is

Water blasting does not give an anchor profile as in abrasive blasting,

hence paint adherence is a problem unless the profile is created by blast-

Another disadvantage of water blasting is the flash rusting. This is a

– by flash blasting the surface after it is dried, or

Surface preparation for organic paint coatings 37

– by using inhibitors in water which can prevent rusting during

hydrojetting.

the alternative is to use special paint systems which are made especially

for hydroblasted surfaces. Flash rusting poses one additional step, while

addition of inhibitor in water is very costly and appears to be less effec-

tive, due to the large volume of water that is used in every cycle, with

more make-up water.

• Another very serious problem is safety, as handling jets at 25 000 to

45 000 psi can be quite dangerous and can cause serious injuries. Hence

specially trained staff are required to carry out water blasting. Of late,

hydrojetting has become much safer as the water nozzles are part of a

plate facing the substrate which slides over the surface by a trolley-type

movement. Of course for uneven surfaces and small areas, jet nozzles

are still in use.

A typical pump which creates a blast of 40 000 psi is more useful for clean-

ing very big structures very fast.

The safer way of hydroblasting is by the use of jet streams, where nozzles

danger from the high-pressure stream to the person handling the nozzle.

Hydroblasting is also very fast.

2.6.1 Water abrasive blasting

Water abrasive blast cleaning uses high pressure water and abrasive to

provide a cleaned and roughened surface. Basically, a small amount of

abrasive is fed into the water stream. The equipment, methods and tech-

nique are similar to those for wet blast cleaning, except that an abrasive

supply is provided for injection into the water stream. The advantages of

wet abrasive blast cleaning include the control of dust emissions and aid in

the removal of chemical contamination by the introduction of water. The

advantages of water abrasive blast cleaning include providing a surface

sive. The limitations of wet abrasive blast cleaning include the accumulation

of considerable amounts of abrasive on the surface because of a lesser

amount of water in and around the abrasive stream, which requires an extra

step of cleaning (washing) to remove the spent abrasive.

2.6.2 Slurry blasting

are mixed together in a pot and blasting is carried out with this wet slurry.

Both the above methods are actually difficult to adopt on site. Thus

profile when compared to the water blast-cleaning method without abra-

A slight modification of water abrasive blasting is where water and sand

Such blasting is quite efficient and provides the anchor profile also.

fitted on the base plate move smoothly on the floor or on the roof, with no

38 High-performance organic coatings

2.6.3 Hydroblasting standards

The steel surfaces produced by hydroblasting do not look the same as those

produced by dry abrasive blasting or slurry blasting. This is because water

on its own cannot cut, or deform steel like abrasives. Hydroblasted surfaces

D steel, with active corrosion pitting, shows a mottled appearance after

hydroblasting. Mottling occurs when the corrosion products are washed out

of the pits, leaving a bright patch, and the surrounding areas are left a dull

grey, brown to black color. ‘Flash rusting’, i.e. light oxidation of the steel,

which occurs as hydroblasted steel dries off, will quickly change this initial

appearance. This standard gives advice on the recommended standard of

application of coatings.

The standard consists of a series of color photographs, which depict the

condition of unpainted steel surfaces prior to and immediately after surface

preparation by hydroblasting. It then shows these surfaces with varying

Initial rust grades [6]

The various rust standards are described in ISO 8501-1:1988 as grades A,

B, C and D. For hydroblasting two initial rust grades C and D are described

in the surface preparation standard and the SSPC VIS-1-89 surface prepara-

• Rust Grade C Steel surface completely covered with rust but with little,

or slight pitting under normal vision

• Rust Grade D Steel surface completely covered with rust, on which

general pitting is visible under normal vision.

Hydroblast standards [7]

The standard illustrates grade C and D surfaces prepared by hydroblasting

to two different degrees of cleanliness. They are designated HB2 and HB2

and are comparable to Sa2 and Sa2

described in the ISO 8501-1:1988

surface preparation standard and to SSPC-SP6 and SSPC-SP10 of the Steel

are given in Table 2.4.

2.7 Summary

Fundamentals of surface preparations are described. Different methods of

pre-cleaning of surfaces, cleaning by hand tools, power tools and blast clean-

tion standard of the Steel Structures Painting Council. The definitions are:

Structures Painting Council surface preparation standards. The definitions

therefore tend to look dull, even before they ‘flash rust’. In addition Grade

hydroblasting and the acceptable level of flash rusting allowed prior to the

degrees of flash rusting.

Hydroblasting

standard

Surface appearance

Light Medium Heavy

HB2 Thorough

Hydroblast Cleaning

The surface shall be free

from visible oil, grease,

dirt and from most of the

rust, paint coatings and

foreign matter.

Light tan-brown

rust will partially

discolour the

original metallic

surface.

Light tan-brown rust may be

evenly distributed or patchy

in appearance, but it will be

heavy enough to mark

objects brushed against it.

Heavy layer of dark tan-

brown rust will com-

pletely obscure the

original metallic surface.

HB2

Very Thorough

Hydroblast Cleaning

The surface shall be free

from visible oil, grease,

dirt, loose rust, paint

coatings and foreign

matter except for

staining.

Table 2.4 Hydroblasting standards and degree of flash rusting

Degree of flash rusting

40 High-performance organic coatings

ing are described in detail. Kinds of abrasives, measurement of surface

namely NACE, SSPC and ISO, are described in detail. Hydroblasting, its

applications, advantages and limitations, are also covered.

2.8 References

1. US document 1100-2-3400, Chapter 7, 30 April 1995.

2. Steelwork Corrosion Control by A. Derek Bayliss and D.H. Deacon, Reinhold

Pub. Corp., 1967, p. 48.

3. Metals Handbook, ASM, 1990, p. 300.

4. www.chlor-rid.com

5. www.spongejet.com

6. Rust Grades, ISO 8501-1, 2007, The Rust Grade Book, LD3020, www.tqc.eu/en/

products/article/68/150-850/

7. Hydroblasting Standards, downloaded from Akzonobel website, www.interna-

tional-pc.com/pc/technical/tech_papers/hydroblast.asp

cleanliness and of anchor profiles and impurities on blasted surfaces are

also discussed briefly. Three important standards for surface preparation,

Techniques for applying organic paint coatings

A S KHANNA,

Indian Institute of Technology Bombay, India

3.1 Introduction

As discussed earlier, coating is both an art and a science. While one needs

to know the chemistry to understand various formulations, application of

coating is an art. What this means simply is to achieve a good, smooth and

protective coating. The skill of the applicator is the most important require-

ment to achieve this. Whether the coating is to be carried out by brush,

coating depend upon how skilled the applicator is.

3.2 Application by brush and roller

Paint coatings can be applied by brush, roller, trawling, air spray gun or

airless spray gun. Application by brush or roller is the simplest. Neither

costly equipment nor a very skilled person is required. However, choice of

a good quality brush or roller of the correct size, and methods to conserve

paint, are some of the important requirements. Brush application is slow;

quality of surface achieved depends upon the fatigue of the applicator.

Because of this, there can be a problem in getting a uniform thickness and

usually increased by 4–5 times. Further, not all paints can be applied equally

by brush or roller. Relatively low viscosity paints can be applied by either,

but high viscosity paints give brush marks.

Before we start discussing spray application, it is important to understand

paint atomization. Atomization is:

• Collection of moving particles on the surface, known as spray

• Spray pattern can be created by variety of means.

As shown in Fig. 3.1(a) and (b), droplets of liquid paint are produced by

interaction of liquid paint with a stream of air or with compressed air.

roller or any sophisticated spray gun, conservation of paint and finish of

finish throughout the surface. Using a roller, the speed of application is

• Breaking up fluid into small particles (droplets)

42 High-performance organic coatings

Atomization is directly related to the viscosity of the paint. A high viscos-

ity paint will produce more atomization than a low viscosity paint as shown

in Fig. 3.2.

There is a very good balance between the quantity of the paint (Q), its

viscosity (V) and the total energy (E) required for atomization. This energy

is provided by the pressure of the compressed gas. This is shown in Fig.

3.3.

3.3 Spray application

There are basically two main types of spray processes: air spray and airless

spray. In the former, compressed air is used to atomize the liquid paint,

while the latter uses hydraulic pressure to atomize the paint. In air spray,

compressed gas with a pressure of 20–100 psi is used, while in airless spray

a pressure of 2000 psi or above is usually required. There are several other

processes:

1. Air spray

2. High volume low pressure (HVLP) spray

3. Airless spray

4. Air-assisted airless spray

5. Electrostatic spray.

3.3.1 Air spray

Air spraying relies on compressed air for paint atomization. Compressed

air from a compressor, connected to the nozzle, breaks the liquid paint into

(a) (b)

3.1 (a) Atomization of liquid drops by a stream of air; (b) atomization

of paint by compressed air.

kinds of spray processes resulting from modification of these two basic

Techniques for applying organic paint coatings 43

tiny droplets that reach the surface and adhere to it. The main component

of an air spray is the spray gun, which is not very expensive. It consists of

a port from where compressed gas is introduced. It is connected to a com-

pressor using a good quality hose long enough to reach the object easily.

Viscosity

Low

Medium

High

3.2 Effect of viscosity on the extent of atomization [1].

Balance

point

Quantity

EnergyViscosity

VQ E

required for atomization [1].

The second port in the gun is connected to a pot filled with liquid paint.

(fluid flow rate)

3.3 Balance between viscosity of paint, quantity of fluid and energy

44 High-performance organic coatings

Compressed air at a pressure of about 40–60 psi forces liquid paint to break

into liquid droplets which have enough pressure to reach the substrate

estimated to be 25–30%. The reason for this is the strong rebound of the

paint droplets from the surface. Many conventional spray guns require 4 l/s

some lacquers, sealers, alkyds and conventional epoxies. It is advisable to

use a separator between the air compressor and the pressure pot, to prevent

moisture and oil from reaching the coating. A schematic of an air spray gun

is shown in Fig. 3.4. A proper gun is held about 18–22 cm from the surface

nozzle and needle size is another way to regulate the amount of coating

spray gun are listed in Table 3.1.

3.4 Schematic of an air spray gun (air pressure 20–60 psi) [1].

Table 3.1 Advantages and limitations of air spray

Advantages Limitations

Fine quality of coating Easy to over-atomize and cause excessive

overspray

Flexible – operator control

of all aspects of atomization

including pressure, fan

Not recommended for protective coating

Inexpensive equipment

Low pressure safety Cannot handle high solid paints

Poor transfer efficiency

surface. The transfer efficiency of this spray process is rather low and is

width, and fluid delivery

of paint flow to operate with a medium viscosity coating such as latex paints,

of the object. By varying the volume of air and coating fluid at the spray

gun, the amount of atomized coating can be regulated. The selection of fluid

exiting the fluid nozzle. Some of the properties of coating achieved by air