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Paint Technologyand practical experience have shown that application of the first coat with a rolleris not to be recommended. This is particularly true on surfaces where the pre-treatment has not been ideal or where moisture or other impurities havingcollected on the surface. Areas generally not suitable to be coated with a rollerare: Complicated structures, porous or uneven surfaces, corners, sharp edges,along areas with many bolt heads or rivet heads.

A paint roller may be used with good results for subsequent coats.

Application by Spraying

A paint brush is not a very practical choice when painting large areas. Here paintspraying equipment is a more effective alternative. The most important advantages ofspray-application are:

It gives a very high production rate and is suitable for large jobs.

All coats, including the first, may be applied by airless spraying.

Airless spraying, when used correctly, offers the advantages of giving ahigher film thickness per coat.

A much more even film thickness is obtained.

The major disadvantages of spray- application are:

It requires a lot of training / practice to apply an even film to a pre-determined thickness, with no defects and a smooth surface.

It is not suitable for application in narrow gaps or areas with difficultaccess (Like rat holes, behind corners etc.)

Location and surrounding environment may be affected.

Airless Spray

High-pressure spraying is used for application of most types of coating used toprotect ships, offshore structures and industrial plants. Paint application usingairless spraying must be carried out correctly in order to obtain satisfactory results.

It is very important that the spray-gun is kept in constant motion.

The spray-gun must be held at the correct distance from the substrate,i.e. about 30-60 cm (12-24 inches).

The gun should be held perpendicular to the substrate

Each stroke should overlap the previous stroke by 50% or crossapplication.

Difficult areas such as profiles, corners, edges, etc. are sprayed first.The other surfaces are subsequently sprayed.

The wet film thickness should be controlled continuously duringspraying.

Always remember to choose the correct nozzle for the job.

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Figure 6.26: Stroke and Triggering with at Spray Application

Start Stroke Pull Trigger Release Trigger End Stroke

Correct distance between spray gun and substrate: (30-60 cm)

Correct angel (90)ange

Overlapping (50% or cross application)

Figure 6.27: Rules for Spray Painting

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(ii) Checks to he carried out during Application

The painter only has one instrument available for checking the film thicknessduring application, wet Film gauge (comb or wheel). It is important to use the camregularly, This ensures good checking during the entire application process so thatthe correct film thickness is achieved over the entire structure. The atmosphericconditions should always be checked during application of paint, and the correct

ventilation ensured, especially in tanks. These conditions must be monitoredthroughout the application process and whilst the paint is drying / curing.

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Substrate

Wet Film Comb Dry Film Destructive Measurement

F i g u r e 6 . 2 8 : D r y F i l mNon-Destructive Measurement

(iii) Mixing and Thinning

Do not add unnecessary quantities of thinner to the paint as this causes slowerdrying and you will have to apply the paint in greater wet-film thicknesses to

achieve the same dry film. Use of the correct (specified) thinners is important asadding the wrong thinner can cause poor results in terms of corrosion protection.

Figure 6.29: Mixing of Paint

When applying two-component paints, the mixing process itself is also important.

Use mechanical agitators and not stirrers. Good agitation or mixing of the two

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components and the correct mixing ratio are important. Follow the instructions onthe technical data sheet for the product. Plural component spray (two-componentspray) are also available for applying special two-component paints, where thecomponents are mixed in the correct ratio in the unit itself.

Figure 6.30: Pre-Blasting Preparation and Application of the

Paint have been Carried out well

6.7 PAINT FAILURES

As you have already studied that paint supplied in containers is a semi-finished product.The finished product only exists once the paint has been applied to the structure in a

complete paint system. This is where we see how good the protection is. The most

important and most common paint failures occur as a result of poor or insufficient

steelwork, preparation or application. Some of these faults are revealed during or shortly

after application but some only appear after a certain period in service. The most

common failures are:

(i)Insufficient film thickness

(ii) Sags/runs

(iii) Dry spraying (iv) Pinholes, popping

(v) Wrinkling/lifting (vi) Blistering

(vii) Rust penetration (viii) Cracking

(ix) Flaking (x) Chalking

( i) Insufficient fi lm thickness is often the result of non-systematicapplication and inadequate checks with a wet film gauge.

(ii) Sags/runs occur when the paint is applied too thickly or too much thinner hasbeen added to the paint. This is probably because the specification has, not

been followed. Occasionally, faults are also found in the paint. The inspector must

note the production number in the daily log. Sags/runs should be repairedimmediately with a brush.

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Figure 6.31: Sags/Runs

Dry spraying is normally a result of poor application or difficult weather

conditions. The most common application fault is too great a distance

between the spray gun and the structure. High temperature combined with

low relative humidity will also contribute to dry spraying as the solvents

evaporate en route from the gun to the object. Strong wind or strongventilation also contributes to an increased risk of dry spraying.

Figure 6.32: Dry Spraying

(iv) Pinholes often occur on porous substrates, for example zinc silicate. On

these substrates, it is important to apply a thin layer of paint, normally called-a

tie coat or by using the mist coat - full coat technique. Pinholes are also

found if there is too strong ventilation during application.

Figure 6.33: Pinholes25

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Figure 6.34: Pinholes Magnified

(v) Blistering is normally an adhesion-related problem and is due to poor cleaningbefore application of the paint. The most common cause of blistering is theapplication of the paint to a substrate contaminated with salt (osmotic blistering).

After blisters have formed, they burst and the underlying unprotected material

begins to rust if rust formation has not already started. Osmotic blistering occurson exposure under water or in areas with heavy condensation. In particular, thesalts sodium chloride and ferric chloride, and welding smoke, cause blistering,Other causes of blistering can be dust or grit on the surface (reduces adhesion),

voids between the steel and the paint or trapped air in the paint film.

Figure 6.35: Blistering

Figure 6.36: Blistering

(vi) Rusting occurs after a blister in the paint film bursts. The failure will occurmost quickly where the paint film is too thin. Particularly susceptible areasare sharp edges, rough welds and places which are difficult to access forapplication. If rusting occurs after a very short time without prior blistering,there will be an opening through to bare steel, i.e. apinhole.

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Figure 6.38: Rusting

(vii) Cracking occurs after a certain ageing of the coating. The causes can be:

The top coat is harder than the coats underneath

Excessively thick system combined with temperature variations

Excessively fast curing of two-component systems

Excessively thick zinc silicate gives "mud cracking".

Figure 6.39: Thick Old Alkyd Paint has been finder Stress

Figure 6.40: Too Thick Paint Film (Cracking)

There are various degrees of cracking. Cracks can either form in the top coat only

or throughout. The time before the fault occurs can vary. Mud cracking occurs,

immediately after application but cracking occurs only after a certain time.

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(viii) Flaking is normally the result of a poorly cleaned substrate (oil, grease) or

the paint being applied onto condensation or surfaces with amine sweating.Paint will frequently flake off from areas with blistering or cracking and

occurs where adhesion is poorest.

Figure 6.41: Flaking

(ix) Chalking is an ageing problem. The binder is degraded by UV radiationfrom the sun and the pigments appear as dust on the surface. The paint'sability to resist chalking will vary according to the binder used. Epoxy willchalk in sunlight after just a few months whereas polyurethane will retain itsgloss for many years.

F igu re 6 .42 : Cha lk ing

(xi) Wrinkling/Lifting

WRINKLING

Appearance

Small wrinkles through or partly through the paint film

Caused by

Skin drying of the paint film, which is usually applied too thick

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Figure 6.43: Wrinkled Surface

L I F T I N G

Appearance.

Caused by

Softening and raising or swelling of a previous coat by the applicationraising swelling by

of an additional coat

Not Normallywhen overcoating A lkyd

Lifting often caused because the solvents in the new coat is too strongLifting

for the previous coat

Figure 6.44: Paint Lifting Defect

6.8 SAFETY, HEALTH AND ENVIRONMENT (SHE)

n recent years. awareness of SHE-related matters has increased. This is reflected in

egislation and by companies and individuals becoming involved in such matters. We

have decided to focus on health problems in connection with the surface treatment of

structures.

Figure 6.45: Safety First

Small wrinkles through the paint film

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General Guidelines

During the preparation and use of paint, it is important to be aware of the risks

associated with the activity. In brief.

Follow current legislation from local and national authorities.

Read the safety data sheets for all products. Know the risks and how to

protect yourself.

Obtain the necessary training in the use of the products and equipment to be

used.

Use the necessary protective equipment to protect the respiratory

system, eyes, skin and hearing.

Ensure correct storage and handling of products and associated

thinners before, during and after the work. The products should be

collected in a specially allocated area.

(i) Safety

Safety aspects are particularly important in connection with the use o

solvent-based paints. Solvents are flamrnable and in the right environment

and under unfavourable conditions can cause both fire and explosion. Beaware that solvents are heavier than air and will sink down to areas which

lie below the painting work.

Figure 6.46: Protective Equipment

In general, the following aspects should be checked in connection with painting work.

Plan the work and co-ordinate this with other activities in the area.

Inspect the area where the work is to be performed. Shut off the area

affected.

Check the equipment to be used, including safety and protective equipment

Closed rooms must be declared gas-free before work begins.gas-free

Take extra safety precautions when painting in tanks and confined area

Adequate ventilation, explosion-proof working lights and equipment,

assistant with a line and adequate breaks in fresh air.