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Why paint galvanized steel?In general, galvanized coatings used alone provide cost-effective long term corrosion protection for steel. Whengalvanized steel is painted it is usually for identity, aesthetics,or added chemical or corrosion resistance in extreme service.

EconomicsCorrectly chosen and applied paint coatings on galvanizedsteel provide outstanding durability and near ultimateprotection for steel. Where steel is exposed to highlycorrosive environments or where access is difficult andfreedom from maintenance is required, duplex systems ofgalvanizing-plus-paint provide very long service life, resultingin outstanding economics.

The longer life of correctly chosen and applied paint coatingson galvanized steel results from the stable zinc substratewhich prevents underfilm corrosion at breaks in the paintcoating, and resists even severe impacts which might

damage a paint film.The synergistic effect of duplexgalvanizing-plus-paint systems

Test results show that suitable paint coatings combined withgalvanizing achieve a synergistic effect in which the duplexcoating may last up to three times as long as the combinedlife of each coating used alone.

Corrosion tests were carried out in the extremely corrosiveenvironment of the Stichting Doelmatig Verzinken WeatheringStation, Amsterdam, Holland, on the North Sea. Under thevery aggressive conditions of the test environment a thingalvanized coating of 100 g/m2 provided four yearsprotection, while a two coat paint system over uncoated steelprovided protection for only three years. But the combination

of an equivalent two coat paint system over the samegalvanized steel provided protection for 10 years.

Painted galvanized steel test panels with heavier 750 g/m2

galvanized coatings were also compared to panels paintedwith an equivalent two coat paint system. The painted steelpanels failed at three years, while the painted galvanizedpanels were still sound after 16 years.

Painting recommendationsfor galvanized steelThe paint ing of galvanized steel requ ires different paintsystems and procedures from those used for paintinguncoated steel. Painting is not difficult but it is important thatthe procedures outlined here and the recommendations ofpaint manufacturers are followed exactly, to ensure long termadhesion, retention of film properties, and optimumperformance.

The tabulated paint ing recommendations given later providecost-effective systems for the following service:

Table 1: Decorative purposes

Rural, urban, mild industrial exposure

Table 2: Decorative purposesModerate industrial, marine exposure

Table 3: Maximum long-term protective lifeHeavy industrial, marine exposure, and possibilityof abrasion

Each of the tabulated recommendations offers the user thechoice of several paint systems. Final choice in each casewill usually depend on prior experience of particular systemsand knowledge of local exposure conditions, taking intoaccount the cost of the paint system and its application.

The following notes provide background torecommendations given in the tables. It is essential that only

compatible paint systems specified by reputablemanufacturers as suitable for galvanized steel are used, andthat the pretreatment, primer, and coating sequence isfollowed exact ly.

Painting galvanized steel

The recommendations given here will produce paint coatings of optimum performance in both functional anddecorative applications. The paint systems recommended are established and proven. They range fromsimple self-priming finishes to complex systems suitable for the most demanding service requirements.Powder coatings over galvanized steel also offer advantages in architectural applications and in severeenvironments.

Premature failure of paint on galvanized steel is usually due to the incorrect application procedures or

choice of paint systems. Adoption of correct systems and application procedures will ensure satisfactoryperformance and in many cases will simplify application. Painting is a complex subject however. Wheredoubt exists, expert advice should be sought.

The following recommendations have been developed in conjunction with Dulux Australia, TaubmansProprietary Limited, AKZO NOBEL Pty Ltd, Jotun Protective Coatings Pty Ltd, and Wattyl Limited.

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Cleaning before paintingAs in all painting operations, the surface to be painted shouldbe clean, dry, and free from grease, oil, and all contaminants.Galvanized steel surfaces are clean and oil free asmanufactured, but should they become contaminated duringtransport, storage or fabrication, the following cleaningmethods should be used:

1 Non-oily soils and dirt should be removed by brushing or

scrubbing. Detergent washing, followed by thorough cleanwater rinsing is satisfactory, provided the detergent is ofthe non-ionic type. Other types should be avoided as theirresidues may interfere with paint adhesion.

2 Remove grease and oily deposits using an alkaline oilemulsifying detergent. Other than for small areas, solventstend to spread oil and grease.

3 Dry thoroughly.

Mechanical preparationMechanical preparation by a light sweep, whip or brushblasting procedure to roughen the surface can provide animproved mechanical key for certain compatible paintsystems but will not improve the adhesion of unsuitable

paints.

Some Australian government departments recommend aprofile height of between 20 and 30 microns, with minimumcoating loss. Brush blasting is covered by AustralianStandard 1627 Part 4.

The following criteria should be observed. Blast pressure of40 psi maximum, abrasive grade 0.2 - 0.55mm (cleanilmenite or equivalent) angle of blasting to surface 45 angle,distance of between 300 to 400mm from the surface, usinga nozzle type minimum 10mm of venturi.

This method requires a high degree of skill and control, andshould be undertaken only by highly experienced operators,to ensure that excessive coating thickness is not removed.

Coating pretreatment forlong-term adhesion

Water-borne self-priming finishesFor less severe exposure, water-borne self-priming finishes ofthe 100% acrylic type give excellent performance ongalvanized steel. They may be directly applied withoutpretreatment other than cleaning.

Water-borne self-priming finishes are ideal for decorative andprotective coatings on new galvanized steel used for exteriorwalls, guttering and roofing. Two-coat application direct to

new clean galvanized steel surfaces is recommended for aminimum of 10 years service.

Near the sea coast. Under more severe exposureconditions near the sea coast, top quality acrylic coatings arenecessary.

An alternative in such exposure is to prime the surface usinga water-borne primer of the type discussed below.

Collection of drinking water. Care must be taken to ensurethat the paint manufacturers specification states that theselected product is suitable for use on surfaces collectingdrinking water. Suitable products include Taubmans SunproofGloss and Satin, and Wattyl Solaguard; however, the firstrun-off should be discarded because of foam from the

surfactant.Maintenance painting of water-borne finishes is normallycarried out using the same type of water-borne finish.

Modified acrylic water-borne primersThese specialised primers provide a simple, easily appliedwater-borne primer for use under water-borne finishes nearthe sea coast and in condensation areas, or for use underother decorative paint systems, including alkyd enamels.

Cold phosphating pretreatmentsSuitable cold phosphating pretreatments for galvanized steel

increase adhesion and greatly enhance the performance ofwater-borne paints in adverse conditions. Their use ismandatory under many of the paint systems recommendedfor more severe exposure conditions.

Cold phosphating pretreatments produce adherent non-metallic conversion coatings of finely crystalline structurewhich provide excellent, permanent adhesion and generallyupgrade the performance of suitable paint systems ongalvanized steel.

Applicat ion usually comprises a three-stage operation. Themanufacturers instructions must be followed exactly.

1 Thoroughly clean and degrease. Alkaline cleaners are notrecommended.

2 The cold phosphating solution is applied by dip, brush or

spray and left for 3 to 6 minutes to allow formation of thegrey zinc phosphate coating.

3 The surface is rinsed thoroughly with fresh water andallowed to dry. Painting should be carried out as soon aspossible after the surface has dried.

Etch primersTwo-pack etch primers or wash primers are ideal for priminggalvanized steel in production line processes where dry filmthickness can be strictly controlled and occupational healthrequirements met. Coatings must be thin, applied by sprayand strictly in accordance with the manufacturersinstructions.

Cold phosphate pretreatment is recommended prior to etchpriming when maximum adhesion is required.

The base vehicle or first pack of a two-pack etch primercontains a polyvinyl butyral resin in alcoholic solvents. Thesecond pack or activator is an alcoholic solution ofphosphoric acid which reacts with the galvanized coating, thepigment and the resin. When applied, the activated etchprimer produces a translucent greenish-yellow filmmolecularly bonded to the surface of the galvanized coating.

Finish coats for severeexposure conditionsFor more demanding exposure conditions, the paint systemslisted will give outstanding performance provided thecomplete system recommendations are followed.

Paints which give good results as top coats with suitableprimers in severe environments include chlorinated rubber,two-pack epoxies, polyurethanes and catalysed acrylics.

Industrially applied heat-cured coatings not covered herewhich also give outstanding results include thermosettingacrylics, silicone modified polyesters and acrylics, andthermosetting vinyls.

Powder coating galvanized steelPowder coating greatly enhances the durability of galvanized

coatings under conditions of severe exposure and hard wear.A wide range of colours and finishes is available .

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Powder coatings are applied by the electrostatic spraying ofdry powders which are then heat fused at moderatetemperatures to form a continuous, homogeneous coating.There is no solvent release during applicat ion or curing,increasing safety and minimising the environmental hazardsoften associated with painting.

In the powder coating of galvanized steel, polyester powdersare the most commonly used, except in undergroundapplications, where epoxies are preferred.

Powder coating should be performed soon after galvanizingand work should not be allowed to become wet beforecoating. Galvanized steel intended for powder coating shouldnot be chromate quenched by the galvanizer.

For normal exposure conditions, the freshly galvanized steelsurface is iron phosphate pretreated, followed by a chromateseal treatment, and then powder coating.

For heavy duty applications where maximum adhesion, saltspray resistance and durability is required, zinc phosphatepretreatment is recommended.

Preparation and sequence of application procedures isavailable on request from Galvanizers Association ofAustral ia. Powder coating hot d ip galvanizing is coveredby BS 6497:1984.

Not recommendedAlkyd enamels and alkyd primers should never be applieddirect to galvanized steel. The practice of weathering beforedirect application of alkyds, for example in the painting ofdomestic roof guttering, will not prevent premature failure ofthe paint coating, nor will chemical etching before painting.

Chemical etching of galvanized surfaces by so-calledmordant solutions based on weak hydrochloric acid,acidified copper sulphate and others must not be used.Etching cannot be controlled and will often remove excessivegalvanized coating thickness, without aiding paint adhesion.

Mechanical preparation is not recommended unless strictlycontrolled as discussed on page 66. Mechanical preparationwill not improve long term adhesion or performance ofunsuitable paints and may result in unnecessary loss ofgalvanized coating thickness.

Weathering of exposed galvanized surfaces as apretreatment for painting is not recommended. The provenpretreatment systems, specialised primers and self-primingpaints for galvanized steel recommended here are easy to

apply and permit more positive quality control.

Painting recommendationsA range of recommendations for painting galvanized steel inthe factory and on site is given in the tables 1, 2 and 3. Threegroupings have been defined and recommendations aremade to cover each condition:

Table 1: Decorative purposesRural, urban, mild industrial exposure

Table 2: Decorative purposesModerate industrial, marine exposure

Table 3: Maximum long-term protective life

Heavy industrial, marine exposure, and possibilityof abrasion

It is essential that the recommendations given are followedexactly. Use of paint types in combinations other than thoselisted, or omission of any part of a recommended systemmay result in reduced performance or complete failure of thepaint film.

In situations of doubt or where specific recommendations arerequired a detailed recommendation should be obtained fromthe specific paint manufacturer.

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Table 2Painting galvanized steelfor decorative purposesExposure condition 2

Moderate industrial and marine exposure

Pretreatment/Primer Primer Undercoat/finish FinishClean, degrease and dryas appropriate. See page 66.

Polyurethane finish

Modifiedacrylicwater-borneprimer

Non-inhibitiveepoxy primer

Cold phosphatepretreatment

PVB self etching primer5-12 m dry film thickness


High build chlorinatedrubber finish

100% acrylicwater-borne finish


Unmodified chlorinatedrubber finish


All finishesNon-inhibitiveepoxy primer

Catalysed acrylic finishHigh build epoxy

Table 1Painting galvanized steelfor decorative purposesExposure condition 1Rural, urban, mild industrial exposure

Pretreatment/primer Primer Undercoat/finish Finish

Clean, degrease and dryas appropriate. See page 66.

Cold phosphatepretreatment *


100% acrylic water-bornefinish *

Alkyd/micaceousiron oxide finish

100% acrylic water-bornefinish *

Alkyd/micaceousiron oxide finish

100% acrylic water-bornefinish

Alkyd finishModified acrylicwater-borne primer *

Alkyd finish

* Required only undermild marine exposure

* Check suitability of paintfor use on surfacescollecting drinking water

* Check suitability of paint

for alkyd finish

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Table 3Painting galvanized steelfor maximum long term protective lifeExposure condition 3Heavy industrial, marine exposure and possibility of abrasion

Pretreatment/primer Primer Finish

Clean and degrease as appropriate.See page 66.

High solids polyurethaneEpoxy primer

Cold phosphate pre-treatment or sweep blasting*

Epoxy finish

All finishes

PVB self etching primer5-12 m dry film thickness

Chlorinated rubber finish

High build epoxy


Catalysed acrylic

Chlorinated rubber finish


* Check paint manufacturersrecommendationsSee page 66

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