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Alkyd paints can only be used above water (not submerged) as the water resistance ispoor. They are not used on zinc primer or galvanised steel as a chemical reaction -saponification - would occur with the binder, with subsequent blistering andflaking. The drying/curing process is also temperature-dependent. This is becausealkyd paints dry or cure by absorbing oxygen from the air. This is a chemicalreaction and such reactions are always influenced by temperature.

The degree of pre-treatment required for the substrate can vary from Sa 2 to Sa 2V2,depending on the purpose of the paint and the environment to which the paint is

exposed. By modifying the alkyds for example with styrene or silicon, otherproperties can be achieved.

(ii) Physically Drying Paints

The group of physically drying paints contains generic types such as chlorinatedrubber (CR), vinyl and acrylic-based paints. These are being withdrawn from themarket due to the high content of volatile organic compounds (VOC). Thechlorinated compounds in CR paints also give off chlorine on ageing.

Physically drying paints are single-component, and dry by pure evaporation of thesolvents. This means that these paints are not so sensitive to the ambient

temperature during application and drying (does not apply to waterborne acrylic).They are also resoluble by other paints which contain strong solvents or in contactwith thinners.

CR paints are used outdoors both above and under water. Vinyl-based paints areused only above water. Acrylic is used as a top coat, as it retains its gloss betterthan chlorinated rubber or vinyl in such systems. Acrylic is also used as primers in

waterborne systems.

(iii) Chemically Curing Paints

Chemically curing paints are thermosetting plastics, unlike physically dryingpaints which are thermoplastics. Thermosetting plastics are more resistant tochemicals than thermoplastics as they form an insoluble three-dimensionalnetwork after curing.

These paints are normally two-component' The supplier provides these paints intwo separate containers, one for the base and the other for the curing agent. Weoften refer to these as component A and component B. Before painting, the twocomponents must be mixed. It is particularly important to mix the components inthe correct ratio and to ensure good agitation. The curing process is a chemicalreaction between the base and curing agent, so application and curing -aretemperature-dependent. It is equally important to apply the paint to the substratebefore the chemical reaction has proceeded for too long after mixing of the

components. We often talk of the usage time (pot life) of paints. When the pot lifehas elapsed, the paint becomes dry and finally completely hard and cannot beapplied.

(iv) Some Chemically Curing Paint Types

(a) Epoxy paints have excellent chemical resistance, particularly to alkalis.They have good adhesion both to steel and concrete and good waterresistance. Epoxy can be modified using phenol, coal tar and hydrocarbonresin to give special properties, e.g. better chemical resistance, betterpenetration, better water resistance etc. One drawback with many epoxypaints is that they contain large quantities of solvent. However, other typeshave now been developed with a high solids content (mastic products) withexcellent "all round" properties. There are also a solvent-free epoxy paints whichare used for drinking water tanks. Waterborne Epox

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Environmental used today because theygive a better working environment. Chemical resistance

Protection however is slightly reduced.

(b) Zinc Epoxy (organic) or Zinc Ethylsilicate (inorganic) are used as cathodicprotective primers on blast-cleaned substrates. Zinc ethylsilicate (solvent-based) and alkali silicate (waterborne) are also often used inside storagetanks for solvents because of the extremely good solvent resistance.

(c) Polyurethane paints are also thermosetting plastics. They are used as topcoats on epoxy ( which chalks in sunlight) as they have excellent weather-

resistance and durable gloss.

(d) Polyester paints are thick coat paints used in areas where a high degree ofwear resistance is required. For example gangways, production decks, damwalls (concrete) for power stations etc. These paints are applied in thickcoats (e.g. 2 x 750 4m) and cure quickly (a few hours). They also have goodchemical resistance.

(e) Vinylester is also a thick coat paint (2x750 m).It has good chemicalresistance and is often used inside storage tanks for chemicals. Such paintscan be used on both steel and concrete.

(v) Antifouling

All surfaces exposed to seawater will be "attacked" by marine organisms. Whenthese organisms attach and grow, they cause a significant increase in surfaceroughness. On a ship's hull, this results in greater friction resistance and henceincreased fuel consumption. This topic is separately dealt with in another unit.

Figure 6.5: Badly Fouled Ship Hull

6.5 PREPARATIONS PRIOR PAINTING

(i) Steelwork

Before a structure is painted, a number of operations must be performed on thesubstrate. The initial work required is generally known as "steelwork". Steelwork isa very important part of the surface treatment and must be carried out before

cleaning and priming of the steel. Good steelwork will ensure that the life time ofthe paint system.meets expectations. In practice, it is impossible to achieve a longlife time for a paint system if the steelwork is omitted or poorly performed. Therequirements for preparation will always be part of the paint specification.

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Figure 6.6: Sharp Edges

Figure 6.7: A well rounded edge will ensure a sufficient

paint film over the entire construction

Figure 6.8: Prior to Pre-Treatment these must be Grinded away

to form an acceptable Substrate for the Paint System

Figure 6.9: Disc Grinding of Weld Beads,

Sharp Edges etc. by means of a Disc Grinder

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Figure 6.10: Grinding of Notch with Rotating File

Steelwork involves the following stages before cleaning and priming:

(i) All sharp edges are rounded to a radius of at least 2 min by grinding.

(ii) All welding beads and slag are grinded off.

(iii) Surface defects such as lamination, etc. are removed by grinding.

(iv) Undercutting in the weld is repaired before priming.

(v) Rough manual welds to be grinded.

(vi) Gas-cut edges are to be grinded before priming.

Pre-Blasting Preparation Laminations, Undercuts, Welding Seams

Figure 6.11: Section Manholes, Well Grinded Edges

F i g u r e 6 . 1 2 : T h e S h a r p E d g e s h a v e b e e n R o u n d e d P r i o r toBlast-Cleaning : Good

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(ii) Pre- Treatment

I has been com pleted, pre-t reatmen t can b egin. T he

purpose of preparation is to ensure that the substrate i s suitable for

application of the paint, i.e. the steel is sufficiently clean and rough.

Contaminants such as of I, grease and salts for example cannot be removed by

blast-cleaning. Before preparation begins, the steel must be properly cleaned.

Cleaning removes contamination and impurities such as oil, grease, salt, dust

and dirt.

Figure 6.13: Removal of Marine Growth, Salts and Loose Paint

by Low Pressure Washing

Salts from a marine atmosphere which are deposited on the structure, and welding

fumes from manual welding are examples of salts which should be washed off before

preparation. Salts can cause osmotic blistering and oil will reduce the adhesion of the

paint. Salts must be removed with plenty of fresh water. Oil and grease cannot be

removed with water alone; strong alkali washing agents and solvents must be used.

Once the substrate is clean, pre-treatment can begin. Here is a brief list of some

methods:

(a) Blast-Cleaning: To ensure maximum paint adhesion, a rough surface is

required. In view of this, blast-cleaning is the best preparation method. Blast-

cleaning removes old paint, rust and scales and gives a clean rough surface.

Possible blast-cleaning methods are dry blast-cleaning, slurry blast-cleaning

(addition of water) and wet blast-cleaning (water with addition of abrasives).

Dry blast-cleaning gives a clean dry surface and the required roughness but

causes considerable dust which contaminates the immediate environment.

Slurry and wet blast-cleaning give a rough, clean surface without dust, but

create flash rust. It has been found that much of the abrasives remain on the

substrate after blast-cleaning. Such contaminants may on some alloys cause a

risk of corrosion at these points. For preparation of stainless steel, aluminium

and galvanised steel, it is important to use nonmetallic abrasives,

Figure 6.14: Corrosion has taken Place Almost the total

area has been Spot Blasted

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Figure 6.15: Surface preparation by spot blasting

(b) Ultra High-Pressure Water Cleaning: This prepara t ion method is

becoming increasingly common. The method consists of removing contamination,

corrosion products and old paint by applying water to the substrate under

extremely high water pressure (up to 2500 bar). The method has two essential

advantages: no cloud of blasting dust is created to contaminate the immediate

environment as in traditional blast-cleaning, and water-soluble salts are removed

from the substrate. It is important to use clean water so that the substrate is

not contaminated by the water used. The method gives a clean surface but will

not give any extra roughness to steel. The original roughness of the steel is

retained where intact paint is removed, but the corrosion pattern on corroded

areas will be considered as roughness where corrosion has occurred. One

disadvantage with water cleaning is that the t endenc y to fo rm fl ash r ust on

the steel wil l inc rease as moisture is added to the substrate. The degree of

flash rust depends on the relative humidity, the temperature of the steel andatmosphere, and the cleanliness of the surface.

Figure 6.16

(iii) Mechanical Cleaning: Use of mechanical cleaning tools such as steel brusl

grinding equipment or machining, does not achieve the same degreecleanliness and roughness as blast-cleaning and the adhesion between

substrate and the paint system will therefore be reduced. Needle guns

example often cause excessive roughness or break-up of the substrate.

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Figure 6.17: Tools for mechanical cleaning

Mechanical rotating wire brush

Needle gun

Hand Wire brush

The method to be used will be described in the paint specification and is primarily

selected on the basis of:

Purpose of the structure

Exposure conditions

Required life time

Restrictions related to environmental requirements and safety.

6.6 PAINT APPLICATION

The protective properties of paint have no effect unless the paint is applied correctly.

Many people think that all the problems are solved as soon as the anti-corrosive paint is

applied in the specified film thickness. It is, however, a fact that the method of

application can be equally important with regard to the final result. The following"

generalguidelines for application are recommended:

All coating systems must be applied in accordance with their productdata sheets.

Application must take place under controlled climatic conditionssuitable for the particular product in use.

If thinning of the paint is required, this must be done in accordancewith recommendations given in the data sheets.

Correct coating intervals, i.e. max./min recoating times according todata sheets, must be respected.

Film thickness measurements must be made after each stage of

application. The wet-film thickness of each coat must be checked and

adjusted, so that the coating thickness meets the requirements in the

specification.

Maximum and minimum paint film thicknesses given by the technicaldata sheets from the paint supplier must be satisfied.

Stripe coating must be done with brush for the first coat. (Rollerapplication can be used on subsequent coats).

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The paint must be applied by means of the most suitable equipment and correct

application technique. Methods of application include the following:wing:

Brush

Roller

Airless spray

Airless Spray: Good

Paint BrushGood, but Slow

Roller: Poor, Particularly

for the First Coat

Figure 6.18: All methods have their good and bad points

(i) Application Technique

Brush Application

From a technical point of view the ideal method of applying a paint is with a

brush. The advantage of brushing is that the paint is worked well into the surface

so that gaps, pores and pits are filled up, for example in uneven welding seams.

Also the paint brush is moved back and forth, moving the paint or coating in two

or more directions. Each of these actions makes the coating flow over the surfacein several directions, increasing the intimate contact of the paint with the surface.

This is particularly valuable where a coating with only low wetting characteristics

is applied.

Another action of the brush is that of stippling or dabbing the coating on to the

surface. This is particularly important when applied around bolt heads, rivets,

welds, and in corners. Such a movement aids in filling pinholes, pits and rough

surfaces with the coating. In many cases the physical action created by using the

brush is the only one that can make the coating flow into these areas. Neither

rolling nor spraying will do this so well.

Type of brushes Application

Flat paintbrush............................................................................................... Large Surfaces

Oval sash and trim brush ..............................................................................Small surfaces

Fitch brush .................................................... .................................................Small surfaces

Oval varnish brush ........................................................... .............................Roughwork

Flat varnish brush..........................................................................................Medium work

French bristle varnish brush .........................................................................High-grade work

Lettering brush ....................................................... .......................................High-grade work

Lettering brush ....................................................... .......................................Small surfaces

Painter's dustes ....................................................... .......................................Cleaning work

Figure 6.19: Types of Brushes and their uses

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FLAT

OVAL SASH TRIM

LETTERING

PITCH

FLAT VARNISH

Figure 6.20: Various types of Brushes

Figure 6.21: A Paint Brush is an Excellent Tool for Stripe Coating

Stripe coat with paint brush:

Where difficult access with spray.

Inside edges and holes

Manual welding seamsg

Comers. angles

Sharp edges

Figure 6.22: Stripe Coating

Coating should be brushed on to all areas which for any reason cannot be properly

spray coated. Surfaces not accessible to brushes should be painted by other

suitable means to ensure a uniforms paint film of adequate thickness.

The main disadvantage with brush application is that the coats will be thinner and

less even than obtainable by spraying. Also the method is time consuming. III

practice therefore we see that the brush is mainly used for:

Touching up areas which are difficult to reach

Stripe coating of edges, corners, weld seams, around notches, etc.

Ideally, a paint brush could be used for the first coat of paint and the subsequent

coats could be applied using a paint roller or., alternatively, by means of

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Stripe Coating

Painting always begins with "stripe coating", Stripe coating means applying an

extra coat of paint on areas where experience has shown that it is difficult to

achieve the specified film thickness by spray. Typical areas which should be stripe

coated are sharp edges, notches, welds (particularly manual welds) and areas

which are difficult to reach with a spray gun. The aim is to ensure good wetting of

the substrate. Stripe coating

should also be used between each coat. A stripe coat

can be applied with a roller on the previous coat where suitable.

Figure 6.23: Note the Contrasting Colours. Excellent Work

Figure 6.24: Proper Striping Before 2d Coat

_________________________________________ T

O

4b,

V011,

Figure 6.25: Proper Stripe Coating

Roller Application

Application by roller is a commonly used method. Rolling is primarily suitable for

areas large flat are such as the exterior of a tank or the flat side of a ship. Experiments2