Upon completion of this lesson, learners will be able to:
Describe various methods of surface preparation
Recognise different preparation tools
Illustrate how surface preparation impacts the performance of coating systems
Methods of Surface Preparation for Outside Shell areas of Ships
This training unit will focus on the surface preparation methods that are used at dry-docking for the outside shell.
The training unit does not cover surface preparation of tanks on the ships.
Methods of Surface Preparation
Taking a ship out of service to undertake remedial painting is very expensive. The Owners loose the charter day rate and have to pay for docking fees, as well as other fixed costs. It is therefore in the Owners interests to keep docking time to a minimum.
Take a minute and think about how long it would take to prepare and paint a ship like this. The total painted hull area is nearly 30,000 square metres.
Methods of Surface Preparation
The average dry-dock period for scheduled dockings varies with location and the extent of work, but it is usually between 5 and 11 days. During that time, all of the outside hull is normally painted.
That includes the topsides, boottop, vertical sides, and flat bottom.
Surface preparation work must start immediately after the dock is dry.
It is usually the most time-consuming and expensive part of the painting process.
Why are Owners willing to pay for surface preparation?
Take a minute and write down the various reasons for carrying out surface preparation prior to painting.
Methods of Surface Preparation
There is only one reason for carrying out surface preparation. That is to enable the paint to bond, or adhere to the substrate. The substrate being either bare metal, or existing intact paint.
To bond properly, it is always necessary to remove surface contaminants from the areas to be painted.
It may also be necessary to roughen or “key” the substrate as well.
The choice of preparation method depends on these factors and also increasingly on “environmental” and “financial” constraints.
Methods of Surface Preparation for Ships
The following methods are widely used for ship repair surface preparation.
High pressure fresh water washing
Degreasing
Hard Scraping
Power tooling
Dry blasting
Hydroblasting (Water jetting)
Slurry blasting
High Pressure Fresh Water Washing
The purpose of washing is:
To remove the salts from all areas, including the topsides.
To remove slime, weed and animal fouling from the underwater hull.
To remove the leached layer of “spent” biocidal antifouling.
This is an illustration of shipyard workers high pressure fresh water washing a ship that has green algae and slime attached to the underwater area.
High Pressure Fresh Water Washing
Most ships are washed manually although some yards have automatic equipment.
The pump pressure should normally be at least 200 bar (2900 psi) to be effective.
The fan jet should be no more than 60 cm (24 inches) from the surface.
Each pass of the fan should be overlapped and the surface washed systematically. It is no use simply waving the jet about and wetting the surface.Washing may be the most important, labour intensive and time consuming preparation operation. Most yards work through the night to complete it, and carry out “pick up” washing in the day light to complete it.
Fresh Water Fresh Water Washing
Dock bottoms are usually washed with sea water from the fire mains. Is this sea water or brackish water acceptable for washing ships?
If you start washing from bottom to top, what happens to the area you have already washed?
Washing at a suitable pressure removes leached layers of antifouling. The photograph shows a typical polishing pattern seen after washing.
High Pressure Fresh Water Washing
This is a foul release coating. They can be soft and can be damaged by high pressure washing.
Pressures should be less than 1,500 (100 bar).
Always seek advice from the paint maker’s technical service representative on the maximum pressure requirements.
The coatings applicators should be briefed before every new project to ensure there is no damaged caused to the coatings and the correct pressures are understood.
Degreasing
Oil and grease contamination have to be removed. Areas are normally limited.
Hard oil or grease is often removed by scraping. However the most common method of degreasing is to spray or roller on a detergent, followed immediately by fresh water washing, as illustrated. Solvent degreasing is rarely used on the outside hull.
If the ship has a foul release antifouling, the entire area may need degreasing, because these coatings may leach silicone oil. Diluted detergents are sprayed on using a paint pump, followed by fresh water washing.
Hand Scraping
Hard scraping is removal of loose paint and animal fouling using a scraper.
The process is slow and labour intensive.
The amount of work, usually estimated on a square metre basis, will not be known until washing is completed. If the ship had extensive barnacle fouling it could be expensive because adherent barnacles are not removed by washing.
There is always a danger of damaging the coatings with a scraper.
Hand Scraping
This is an illustration of ship with intact barnacles after the high pressure fresh water washing.
Thorough scraping will be required to remove the barnacles.
Some of this scraping will inevitably damage the coating and barnacle bases will not be removed.
Scraping
There will always be some hard scraping to do in sea chests as illustrated which are prone to animal fouling.
Sea chest are recesses in the hull that take in water for ballast tanks and engine cooling water.
Power Tool Cleaning
Power tooling is used as an alternative to spot blasting when:
There are environmental, or technical restrictions on blasting.
When corroded areas are small and scattered.
When yards set high “minimum” blast areas.
Power Tool Cleaning
Disc grinders are generally used for power tooling.
They are very good at feathering edges (Spot blasting is not good at feathering)
They don’t produce over-blast damage (Spot blasting does)
They do not “increase” the repair area (Spot blasting massively increases touch up areas)
They do not produce a surface profile on the steel, which means repairs do not perform as well as spot blast repairs.
Orbital Disking and Sanding
This animation demonstrates the correct method of a spot coating repair.
If there are many spot repairs on a surface the task will be very labour intensive to feather edge all the spots.
After the preparation has been conducted the paint should be correctly overlapped.
The recommended overlap for spot coats is 50mm (2'') per coat.
Bristle Blaster
Mechanical tools are available which can remove all the corrosion and coatings to a standard near blast cleaning. A bristle blaster is common in new build ship building and on certain ship maintenance, especially if a high performance coating is to be used.
Surface Preparation
The bristle blaster is ideal for small works including coatings removal. A bright clean surface is achieved leaving a surface with a surface profile, albeit not identical to to an abrasive blast cleaned surface.
This equipment will clean welds thoroughly as opposed to mechanical grinding and disking which will not penetrate the weld configuration.
Surface Preparation
This video demonstrates the use of a bristle blaster on a coated surface.
This equipment rotates in a similar fashion to a mechanical wire brush but the specially designed wires are propelled onto the substrate.
This method has the advantage of creating a surface profile on the substrate along with a thorough cleaning in and around the weld.
Dry Abrasive Blast Cleaning
Full blasting is carried out to either Sa 2*, or Sa 2½** standards.
* NACE No.3/SSPC-SP 6
**NACE No.2/SSPC-SP 10
Areas for full blasting are agreed by the superintendent and paint manufacturers representative after docking and washing when all of the hull is visible .
Blasting may take several days for large areas, which may “turn” (ginger), requiring a re-blast. Alternatively holding primers may be used during a “blast – prime” “blast – prime” operation.
Dry Abrasive Blast Cleaning
Sweep blasting is often specified as an alternative to full blasting.
Hard sweeping usually means the removal of most of the intact paint.
Light sweeping (often referred to wrongly as Sa 1 standard) leaves most of the paint in place.There are no widely recognised visual standards for sweep blasting. Agreement is usually reached by blasting a trial patch before the main operation starts.
Dry Abrasive Blast Cleaning
Blasters cannot control the abrasive stream without the use of dead man’s handles. This leads to:
Uncontrolled blasting.
Large amounts of over-blast damage.
Non feathering of edges.
Spot blasted areas are always much larger than the original area of corrosion/breakdown. At its best, spot blasting is a quick and easy way to remove corrosion. At it’s worst it can produce surfaces that are unfit to paint and will break down in service.
Note: Not all shipyards use a dead man's handle when blasting the outside of a shell as the operative can be seen by the assistant.
High Pressure Water Jetting
There are increasing environmental constraints on the use of dry blasting, mainly because of the dust pollution it causes. Wet blasting techniques have therefore become more common.
High pressure water jetting uses high pressure water to remove corrosion and intact paint. Pressures may be in excess of 2,500 bar (36,000 psi). It is therefore a very dangerous technique and should only be carried out by fully trained operators.
High Pressure Water Jetting
High pressure water jetting is slow and expensive compared to dry blasting, but it does produce a much cleaner surface with no salt or dust contamination.
It can be used for both full and spot blasting.
It is often specified for repairing foul release products where dust contamination may be an issue.
High Pressure Water Jetting
High pressure water jetting results in a “Flash Rusted” surface. Flash rusting is quantified either as “light”, “moderate” or “heavy”.
Most Paint Makers will accept either light or moderate flash rusting for products used on the outside hull.
Heavy flash rusting is usually removed by washing. The wet area then dries to a light or moderate standard.
Water Jetting
This animation illustrates the removal of a coating on a steel surface.
There is little surface profile underneath the coating.
Water jetting will not produce a surface profile with water alone.
It will, however, re-produce an original surface profile.
WaterJetting Standards
Waterjetting standards have been developed for the surface preparation industry;
SSPC-VIS 4 /NACE VIS 7
ISO 8501-4
It is important that the contractor (coatings applicator) is aware of which standard is being used for the contract.
The two documents are visual documents and detail the different levels of waterjetting and flash rusting.
USA refer to waterjetting and ISO refer to water jetting
Wet Abrasive Blast Cleaning (Slurry Blasting)
Slurry blasting uses a mixture of abrasive and water for blasting. Various types of equipment are available.
Shroud blasting. This uses a simple water nozzle fitted to a dry blast nozzle.
Entraining abrasive directly into a high pressure water jet.Slurry blasting can be used for both full blasting and spot blasting.
Wet Abrasive Blast Unit Set Up
The fresh water is fed up the hose and mixes with the abrasive at the nozzle. This is generally termed 'slurry blasting' in certain industries.
Some units spray the water into the abrasive airsteam as it leaves the nozzle.
The water line is usually attached to the abrasive blast hose (illustrated in dark blue) before entering the nozzle in the side wall.
It is possible to mix the water with the abrasive in the blast pot. This process uses less water and is more suitable for spot repairs and general control of the blast cleaning.
Wet Abrasive Blast Cleaning
This is a typical spot blasted area on a ships hull during dry docking.
The process is good for feathering of edges and there is little over-blast damage.
A surface profile will be produced and generally all soluble salts are removed.
Wet Abrasive Blast Cleaning
Advantages:
It can reduce dust pollution and the amount of abrasive used.
Some equipment is good at reducing over-blast damage and feathering edges.
Wet abrasive blast cleaning will produce a surface profile.
Other people can work in the dock when blasting is in operation.
It is quicker and cheaper than high pressure water jetting.
Soluble salts will be removed.
Wet Abrasive Blast Cleaning
Disadvantages:
Wet abrasive sticks to the blasted surface and must be removed by high pressure washing. This adds another operation to the schedule.
The surfaces flash rusts as with high pressure water jetting.
Water Jetting and Wet Abrasive Prepared Substrates
Due to the increase in the use of water jetting and wet abrasive blast cleaning as a method of surface preparation, coating systems have been developed which can be applied onto the oxidized or gingered surfaces.
The leading suppliers have spent considerable time and resources in producing and testing systems that can be used with water jetting and wet abrasive blast cleaning.
Heavy flash rusting must be avoided.
A hard bristle brush and a wash-down are usually utilised for this process.
Summary
Within this training unit we have reviewed the various forms of surface preparation which are conducted in ship repair and maintenance during dry docking. These included;
High pressure water washing
Degreasing
Hand scraping
Disking/abrading
Water jetting
Wet abrasive blast cleaning
Surface preparation for ship new build will be covered elsewhere in the training.
