Amtec Guide to Surface Preparation

 

Good surface preparation maybe considered to be the most  important part of the entire coating process, in that the greatest percentage of coating failures can be traced directly to poor surface preparation.  All paint systems will fail prematurely unless the surface has been properly prepared to receive the coating.  If contaminants such as loose rust, oil, grease, dirt, salts, chemicals, dust, etc. are not removed from the surface to be coated, adhesion will be compromised and/or osmotic blistering will occur in addition to premature failure of the coating in service.  No paint system will give optimum performance over a poorly prepared surface.

 

Surface Cleanliness.

The extent to which a surface is made clean before the coating is applied, is a balance between the expected performance of the coating, the paint manufacturers recommendations, the time available for the job, the relative cost of the various surface preparation methods available, access to the area to be prepared and the condition of the steel prior to surface preparation.  In many instances, coatings cannot be applied under ideal conditions, especially under repair and maintenance conditions.

The quality of surface cleanliness which is achieved (or which it is possible to achieve) will be very different for an un-corroded high quality steel plate with tightly adherent millscale, arriving at a new building shipyard and steel on a vessel which has been in service for ten years, with poorly adherent coating, loose rust scales and heavy pitting.

Any substance which prevents a coating from adhering directly to the steel can be considered a contaminant.  Major contaminants at new building include:

• Moisture or water.
• Oil and grease.
• Ionic species from the nearby sea and industrial areas.
• White rust (zinc salts from weathered zinc silicate shop primers).
• Weld spatter.
• Weld fume.
• Cutting fume.
• Burn through from welding on the reverse side of the steel.
• Dust and dirt from the yard site and from neighbouring industrial processes.

 

In addition, at Maintenance and Repair situations, the presence of pitting, corrosion products, cathodic protection products, aged coatings and trapped cargoes, etc must also be considered, particularly if only localised surface preparation of the most severely affected areas is being carried out prior to recoating.  This is particularly important for outer hull refurbishment, water ballast tank and cargo tank coating repairs.

Photograph1.  Outer hull showing application of primer after localised spot blasting.

 

Surface Cleaning and Conditioning.

There are many methods currently available for the cleaning and preparation of steel surfaces prior to painting.  The choice of preparation method will depend upon the areas of the vessel to be prepared and the equipment available.  For example in a dry dock, the outer hull may be prepared by abrasive or water blasting before paint is applied. 

At new building, water ballast tank blocks may be prepared for coating by thorough abrasive blasting to a pre-agreed standard such as ISO 8501 Sa 2.5, or by sweep blasting (a light blast which does not remove all the intact shop primer) or by power tools.  On board maintenance may involve abrasive or water (hydro) blasting (jetting), power and/or hand tool preparation depending upon the size and location of the area to be prepared and painted.   As with edge preparation, a higher standard of surface preparation, will result in a longer and more effective coating performance.

 

Soluble salt removal

One of the major causes of coating blistering is the presence of retained soluble material, such as salt, on metal surfaces before painting.  For some types of paint, such as chemical, cargo and ballast tank coatings, the level of soluble salts present is crucial to the long term performance of the coating. 

Whilst salts are easily removed from flat surfaces by water washing, it is the salts which become trapped in cracks in the coating, under old paint and rust and in pits in the steel surface. Such residual salts will cause blistering or detachment of the new coating, if not removed.  High pressure water washing will remove the majority of these trapped salts, if carried out effectively.

Paint manufacturers specify the level of soluble salts which may be present on the surface before coating application and these will vary depending upon the paint itself and its service environment.

 

Solvent Cleaning

 

Photograph2. Oil & grease stain stains during block assembly show as dark stains on the shop primer.

 

This is a process of using solvents or other cleaning compounds, to remove oil, grease and other similar contaminants.  This process is best utilised as a preliminary step in the total surface preparation procedure, since subsequent cleaning processes, such as abrasive blasting, may simply spread some of the contaminants more thinly over the surface rather than completely removing them.

Solvents are not the preferred cleaner recommended by paint companies for large areas of contaminants, as they may become an impediment rather than a help if not properly removed.  A proprietary, water soluble, oil and grease remover followed by copious fresh water washing is the preferred method of achieving this standard.  Care must be taken that the cleaner does not leave any ionic residues on the surface, particularly if the fresh water washing is limited to the use of buckets of water and cloths.

If solvent cleaning is chosen, then safety is very important.  Adequate ventilation and minimising the potential fire hazard are paramount. Clean-up rags should be changed often to prevent smearing and two or three solvent applications may be necessary.  Brush application should be avoided, or the oil is simply spread over a larger area, as shown in the photograph.

Photograph 3.  Poor removal of oil contamination using a brush.  The dark area at the top of the photograph shows the extent of oil contamination after cleaning.

 

Abrasive blasting.

This is the most commonly used method of preparing a surface for the application of paint.  When properly carried out, abrasive blasting removes old paint, rust, salts, fouling, etc and provides a good mechanical key (blast profile) for the new coating. 

After abrasive blasting is completed, the surface must be cleaned to remove loose debris and dust before painting commences.

If too high a blast profile is produced, inadequate coating coverage will result over any high and sharp peaks and this could lead to premature coating breakdown.  However, abrasive blasting can also result in an insufficient surface profile and may also simply re-distribute contamination over the steel surface trapping contaminants under the surface as shown in the diagram below. 

.  Contamination trapped in the blast profile will be overcoated.

 

If the blasting media is contaminated, the quantity of soluble salts on the steel surface after blasting can be higher than before blasting.  The quantity of soluble salts in blasting media can be checked by aqueous extraction techniques.  Soluble contaminants remaining on a surface can be quantified using commercially available tests.  Coating manufacturers representatives and relevant ship yard personnel could advise on these tests when necessary.

 

Abrasive Blasting Media.

There are many types of abrasive blasting media available and each has its own characteristics in terms of shape, profile produced and recycling properties, etc.  The choice of blasting media will depend upon local availability and the desired results.  It is not possible to list all the blast media here, but common examples are:

 

Type of Abrasive	Mesh size	Typical max. height of Profile
Very fine sand	80	37mm (1.5 mils)
Coarse sand	12	70mm (2.8 mils)
Iron shot	14	90mm (3.6 mils)
“Copper slag” (1.5-2.0mm grain size)	-	75-100mm (3-4 mils)
Iron grit (G16)	12	200mm (8 mils)

Table1. Profiles produced by different abrasives.

Specialist blast media – e.g. garnet, etc, are also available.  Garnet can be used where disposal of used blast media is difficult (for example during on board maintenance), as it can be recycled several times without loss of blast quality and does not decompose into rust (compared with iron grit) if it becomes damp, thus aiding its removal from the interior spaces of tanks.

 

Abrasive blasting advantages.

• Can be used to clean large areas of steel.
• Gives a good profile to steel before painting.
• Removes rust, old paint, some oils, grease & soluble salts.

 

Abrasive blasting disadvantages.

• Environmentally unfriendly – noisy, dirty, produces large quantities of dust.
• Can leave retained grit in the steel which is not fully overcoated.  The grit breaks down early in service giving the appearance of a poor coating performance.

 

Spot blasting.

An abrasive, localised preparation process commonly used on the outside of ships hulls during repair and maintenance work, when patches of localised corrosion have occurred.  Care must be taken to avoid the following problems:

• Undercutting and loosening of paint edges around the cleaned spot.  Edges must be feathered, where possible.
• Stray abrasive particles will damage surrounding paint in confined spaces and this must be treated and repaired as necessary.
• Blasting should be discontinued whilst moving from one spot to the next rather than trailing blast media over the surface.  Any damage caused in this way should be repaired.

 

Sweep blasting

 

A jet of abrasive is swept across the surface of the steel rather than being focussed on one area for any period of time.  Its effectiveness depends upon the type and particle size of the abrasive used, the condition of the surface and the skill of the operator.  Three major types of sweep blast are in common use:

• Light sweeping: is used  to remove surface contamination or loose coatings.  It is also used for etching of existing coatings to improve adhesion.  Fine abrasive (0.2-0.5mm) is commonly used for etching.
• Heavy or hard sweeping: is used to remove old coating, rust back to the original shop primer or bare steel.
• Sweeping Shop Primers at new building: is used to partially remove the shop primer to an agreed standard, immediately prior to overcoating. 

 

Hydroblasting /  Water Jetting.

Whilst dry abrasive blasting is the most commonly used method of surface preparation,  Government and local regulations are continuously changing and require the development of more environmentally sensitive and user friendly methods of surface preparation and the use of hydroblasting (also known as hydro jetting, water blasting and water jetting) is becoming an increasing viable means to accomplish this.  Standards are being developed to satisfy this need. 

It should be noted that hydroblasted surfaces are visually very different from those produced by abrasive cleaning or power tools and surfaces often appear dull or mottled after the initial cleaning is completed.

One drawback of hydroblasting is the formation of flash rust (also called flash back or gingering) after blasting.  Heavy rust formed in a short time period is indicative of residual salt on the steel and re-blasting is necessary before painting.  Light rusting is generally acceptable to the paint manufacturers, subject to the coating to be applied and the area in which it will be used.

Hydroblasting does not produce a profile on the steel surface as compared with abrasive blasting.  It does however remove rust and loose paint, as well as soluble salts, dirt and oils, from the steel to expose the original abrasive blast surface profile plus the profile produced by corrosion and mechanical damage.  The use of ultra high pressure hydroblasting can also remove adherent paint from steel. 

The terms water washing (usually used to remove salts, slimes and light fouling from vessels in dry dock) and hydroblasting (used to remove rust and paint) can easily become confused.  To clarify the situation, the following pressure guidelines are given:

Low pressure water washing/cleaning – pressures less than 1,000 p.s.i. (68 bar).

High pressure water washing/cleaning – Pressures between 1,000 & 10,000 p.s.i. (68-680 bar).

High pressure hydroblasting – pressures between 10,000 & 25,000 p.s.i. (680-1700 bar).

Ultra high pressure hydroblasting – pressures above 25,000 p.s.i. (1700 (bar).  Most machines operate in the 30,000 –36,000 p.s.i. (2000-2500 bar) range.

Inhibitors can sometimes be added to the water to help prevent flash rusting prior to coating being applied, however they are often ionic in nature and must be completely removed by further washing before the paint is applied.  It is also important to ensure that the water being used should be sufficiently pure that it does not contaminate the surface being cleaned.

 

Advantages of hydroblasting are:

• Removes soluble salts from the steel surface.
• Water as a cleaning material is generally inexpensive and available in large quantities.  (The Middle East is an exception).
• Lack of contamination of surrounding areas because there are no abrasive particles.
• Lack of dust.

 

Disadvantages of hydroblasting are:

• No surface profile is produced.  It relies on the original profile, if present.
• Flash rusting may be a problem in humid environments.
• The areas behind angles require particular attention as they are difficult to clean by hydroblasting.

 

Power Tool Cleaning

A pictorial series of standards showing welds and damage on shop primers both before and after different types and grades of cleaning, is produced by the Japan Shipbuilding Research Association.  ISO standards for power tooling are also available, based on several different steel grades.

The effectiveness of cleaning using power tools rather than abrasive or water blasting methods will depend on the effort and endurance of the operator, as working above shoulder height is especially tiring.  Some of the more popular methods are as follows:-

Rotary Power Discing.

This one is the most commonly used surface preparation methods for the majority of on-board maintenance situations.  It is also widely used at new building for the preparation of welds and cut edges prior to painting.  Normally silicon carbide discs are used and the grade selected to suit the conditions of the surface to be abraded.  It is important to change the discs at regular intervals in order to maintain efficiency. 

Care should be exercised in the selection of the grit size and type of disc to be utilised, so that the surface is not excessively smoothed, thereby reducing the ability of the paint to adhere.  Irregular and pitted surfaces may require a combination of the various power tool cleaning methods to maximise effectiveness.

Power disc preparation is also widely used in shipyards for edge and weld preparation and is used in some shipyards at new building for the preparation of all block surfaces prior to coating (as blasting is only used for plate preparation prior to shop primer application).

Mechanical Descaling.

Needle Guns, Roto-Peen, and other pounding type instruments are effective to some degree in removing thick rust and scale and are frequently used by ships crew for maintenance of vessels in service.  The action of these types of devices is dependent upon cutting blade or point pounding the surface and breaking away the scale.  Cleaning is only effective at the actual points of contact.  The intermediate areas are only partially cleaned, because the brittle scale disintegrates, but the lowermost layer of rust and scale remains attached to the substrate. 

Rotary Wire Brushing

This method has some merits, depending upon the condition of the surface.  Loose “powdery” rust can be removed but hard scale will resist the abrasion of the wire bristles.  When rust scale is intact and adherent to the substrate, rotary wire brushing tends to merely burnish or polish the surface of the rust scale, but does not remove it.  Care should be exercised, in that the burnished surface may give the appearance of a well cleaned surface, which is often misleading.

Hand Tool Cleaning

This method is the slowest and usually the least satisfactory method of surface preparation.  It is frequently used in confined areas where power tool access is not possible.  Scrapers, chipping hammers or chisels can be used to remove loose, non adherent paint, rust or scale but it is a laborious method and very difficult to achieve a good standard of surface preparation.  Wire brushing can make the surface worse by polishing rather than cleaning the rusted surface.  Soluble salts, dirt and other contaminants are frequently trapped and overcoated, leading to early paint breakdown. 

Pickling

An acid pickling process can be used for the preparation of small items before coating.  The items, such as pipes, are alkali cleaned followed by a wash and then an acid pickling bath to remove rust.  Through washing must take place to remove all the acid, particularly if the item is to be painted.

 

Preparation of Non-Ferrous Metals.

Galvanised steel

The surface must be dry, clean and free from oil and grease before painting.  Degreasing requires some effort to obtain a clean surface, as the zinc corrosion products can trap grease and other contaminants.  Any white zinc corrosion products should be removed by high pressure fresh water washing or fresh water washing with scrubbing. 

Sweep blasting or abrading are suitable preparation methods, but fresh water washing should be used additionally to remove soluble salts.  An etch primer can also be used after cleaning to provide a key for further coatings.  Paint companies should be consulted on suitable preparation methods, primers and coatings for galvanised steels and will advise on individual cases.

Aluminium

The surface should be clean, dry and free from oil and grease.  Corrosion salts should be removed by light abrasion and water washing.  Clean surfaces should be abraded or very lightly blasted using a low pressure and a non-metallic abrasive (e.g. garnet).

Alternatively, a proprietary etch primer should be used to provide a key for subsequent paint coats.  Paint companies should be consulted regarding suitable primers and coatings.

 

Edge and Weld preparation at New Building.

Experience has shown that edges and welds are generally the first areas to show corrosion and coating breakdown, particularly in ballast and cargo tanks.  This is due to a number of inter-related processes including surface preparation, coating application, deflection, shear and buckling stresses on the edges and welds and so on.  The quality of the surface preparation will play a major role in determining the service lifetime of the coating.  Ring stiffeners around openings may reduce this effect.

Weld Preparation.

The process of welding generally produces some type of slag on the weld itself, together with spatter (small droplets of the parent or weld material) and fume (smoke).  Submerged Arc Welding (SAW) does not generally produce welding fume as the arc is covered by a slag blanket.  Slags may have to be removed manually.  

Weld fume (brown stains) & spatter.

Removal of the weld spatter is essential as this material will cause an irregular surface and will result in poor coverage by the paint.  Spatter is adherent and must be removed by chipping or other mechanical means. 

Weld fume must also be removed as it is loosely adherent to the steel and depending upon the type of weld consumable used, the fume may contain water soluble species.  If weld fume is overcoated, the paint may blister and/or peel from the steel in service.  Blisters can also form where the shop primer is damaged due to the welding of stiffeners on the other side of the plate.  This is often referred to as “burn through” and the photograph below is typical of the results in service when the burn through is not adequately cleaned or removed prior to coating application.

Blistering where a stiffener was welded on the other side of the plate.

 

The position of welds can present difficulties to the cleaning and surface preparation process, particularly when the weld forms part of a complex structure such as the bulbous bow on a forepeak ballast tank.

 

Complex structure, with lower welds only accessible through the hole.

 

In many cases, only an abrasive or water blasting process will provide an efficient cleaning of the weld, however this may not be practical in some new building shipyards or for on board maintenance of vessels in service.  Welds must be prepared efficiently so that the possibility of creating voids under the coating is eliminated.  Porosity can also occur in welds and this may not become visible until the weld has been blasted clean at the new building stage.

Schematic fillet weld showing typical defects which form voids under the coating.

 

Edge Preparation.

After application whilst the coating is still liquid, there is a tendency for many coatings to pull back from sharp edges leaving a very thin layer of paint which can quickly breakdown in service.  Grinding profiles into the edges of cut outs, drainage holes, etc, as shown in the figure below, greatly improves the adhesion and coverage of the coating around the edge.  Rounded edge preparation will generally provide the most effective service performance from the coating.  Three passes of the grinding disc over the cut edge will give the next best preparation, then two passes.  Even one pass of the grinding tool will give a better surface for painting  than no preparation.  The addition of a stripe coating to the edges (as discussed earlier) is also beneficial in providing long term protection.

 

Types of edge profile.

 

Good surface preparation around cut edges is also very important.  Cutting fume is an additional source of contamination at edges and this must be removed for good coating adhesion. Ring stiffening around manholes maybe proven beneficial for the longevity of the coating around the opening.

 

Surface Preparation Standards

The following table provides a summary of Surface Preparation Standards and a cross-reference of those Standards by various world-wide agencies.  There are differences which can be important in some instances and care is advised when a crossover is required.  While these standards are limited to steel substrates, many of the techniques, with their inherent advantages and disadvantages, hold true for other substrates, although advice should be sought directly from the individual coatings manufacturers when overcoating other metals such as galvanised parts or stainless steels.

 

Summary of Visual Preparation Standards.

Cleaning method	Description (Summary)	Swedish / ISO	SSPC	JSRA	NACE
Abrasive blast	White metal/visually clean	Sa 3	SP 5		1
				Sh/Sd 3
	Near white metal/very thorough clean	Sa 2.5	SP 10		2
				Sh/Sd 2
	Commercial/thorough clean	Sa 2	SP 6		3
				Sh/Sd 1
	Brush/light blast	Sa 1	SP 7		4
Hydroblast			SP 12		5
	Sa 2.5 equivalent	**
	Sa 2 equivalent	**
Power tool	Very thorough cleaning	St 3	SP 3	Pt 3
	Thorough cleaning	St 2	SP 2	Pt 2
Hand tool	Very thorough cleaning	St 3	SP 3
	Thorough cleaning	St 2	SP 2

Comparison of visual preparation standards.

** = An ISO standard is in preparation, based on the International Coatings standards.
ISO = ISO 8504:1992(E) and 8501-1:1998(E) and 8501-2:1994
Swedish = SIS 05 59 00 (1967)
SSPC = Steel Structures Painting Council (now the Society for Protective Coatings). 
JSRA = Japan Shipbuilding Research Association – Standard 1984
Where: Sh = shot blasting
Sd = sand blasting
NACE = National Association of Corrosion Engineers

 

Standards for Abrasive Blast Cleaning

It should be noted that the majority of the standards such as ISO or JSRA for steel preparation, are based on a visual assessment of the surface condition only.  Note that it is possible for steel to appear visually clean whilst a sufficiently high level of soluble salt contamination remains on the surface, to cause blistering of the paint when the vessel is in service.

The following standards are used for judging the surface cleanliness of steel are based on visual observations only and are all assessed relative to the original condition of the substrate prior to cleaning.  Steel is usually categorised into four grades - A, B, C and D grade, where A grade is in the least corroded condition.

ISO Sa 3 Blast cleaning to visually clean steel

When viewed without magnification, the surface shall be free from visible oil, grease and dirt and shall be free from millscale, rust, paint coatings and foreign matter.  It shall have a uniform metallic colour.

ISO Sa 2.5 Very thorough blast cleaning

When viewed without magnification, the surface shall be free from visible oil, grease and dirt and shall be free from millscale, rust, paint coatings and foreign matter.  Any remaining traces of contamination shall show only as slight stains in the form of spots or stripes.

ISO Sa 2  Thorough blast cleaning

When viewed without magnification, the surface shall be free from visible oil, grease and dirt and from most of the millscale, rust, paint coatings and foreign matter.  Any residual contamination shall be firmly adhering.

ISO Sa 1 Light blast cleaning

When viewed without magnification, the surface shall be free from visible oil, grease and dirt and from poorly adhering millscale, rust, paint coatings and foreign matter. 

 

Hydroblasting / water jetting standards.

The major paint companies have worked for many years with the hydroblasting technique to produce standards for use with their products.  A typical example is the hydroblasting standard from International Coatings Ltd.  An ISO standard on hydroblasting / water jetting will be produced in the near future.

International Coatings hydroblasting standard for C grade steel.