Section 3 Surface preparation, application and maintenance of coatings

3.1.1 These notes have been prepared to give general guidance on those aspects of surface preparation and application and the subsequent maintenance of coatings that should be taken into account by those agreeing the coating specification.

3.1.2 These notes are not intended to be used for contractual purposes or as representing the minimum requirements as these are a matter for the interested parties to agree.

3.1.3 The guidelines do not intend to replace the technical aspects of any specific coating system, to be covered by the product and job specifications, which are at the discretion and under the responsibility of Owners, manufacturers and construction yards.

3.1.4 Owners should select and maintain a corrosion protection system to ensure an adequate level of protection.

3.1.5 Coating manufacturers should give evidence of the quality of the product and its ability to satisfy the Owner’s requirements.

3.1.6 Coating manufacturers should have products with documented service performance records. Coatings recognised by Lloyd’s Register (LR) are considered as satisfying this requirement, see list of LR approved PSPC compliant coatings on Class Direct. Where it is proposed to use coatings without satisfactory performance records, coating selection should be supported by appropriate laboratory test data carried out in accordance with recognised Standards (e.g. ISO 20340) in order to verify their suitability for the intended service condition.

3.1.7 The construction yard and/or its subcontractors should provide clear evidence of their experience in coating application. The coating standard, job specification, inspection, maintenance and repair criteria should be agreed by the construction yard and/or its subcontractors, Owner and manufacturer.

3.2.1 At present, hard coatings are the most commonly used for new construction.

3.2.2 As their effectiveness and life are influenced by several factors it is essential that the manufacturer’s technical product data sheet and job specifications are followed.

3.2.3 Multi coat applications with coating layers of contrasting colours are recommended. The last coating layer in ballast tanks should be of a light colour in order to facilitate in-service inspections.

3.2.4 Measures should be adopted at the design stage to reduce scallops, use rolled profiles (provided this does not adversely affect fatigue performance) or three-pass grinding where possible and ensure that the structural configuration permits easy access for personnel and equipment and facilitates cleaning, draining and drying of tanks.

3.2.5 Where a coating is supplemented by cathodic protection, the coating must be compatible with the cathodic protection system.

3.3.2 Coatings intended for use underneath solar heated decks or on bulkheads forming boundaries of heated cargo or fuel oil spaces should be able to withstand constant or repeated heating without becoming brittle or subject to a loss of adhesion. Due regard should be given to the possible poor edge-covering properties of hard coatings with a high solid content.

3.4.1 Tubular scaffolding should not mask surfaces to be coated. Where contact is necessary, spade ends should be used.

3.4.2 Staging should afford easy and safe access to all surfaces to be coated.

3.4.3 Tubular scaffolding should be plugged or capped prior to blast cleaning to prevent the ingress of grit and dirt.

3.4.4 Staging should be designed to allow thorough cleaning.

3.4.5 Staging layout should be such that ventilation is not rendered ineffective.

3.4.6 Care should be taken when removing scaffolding in order to keep damages to a minimum. Any damages should be repaired in accordance with the paint manufacturer’s recommendations.

3.4.7 External surfaces of pipelines which will be covered by pipe clips should be blasted and coated prior to fitting.

3.4.8 Pipeline exteriors should be blasted and coated at the same time as the lowermost parts of the tank. Any overblast or over-spray affecting surrounding areas should be repaired.

3.4.9 Lighting during blasting and painting must be electrically safe and provide suitable illumination for all work.

3.4.10 Powerful spotlighting must be provided for inspection work.

3.4.11 Adequate ventilation during application and drying of all paints is essential.

3.4.12 Flexible ventilation trunking should be used to allow the point of extraction to be reasonably close to the applicator.

3.4.13 The ventilation system and trunking should be so arranged that ‘dead spaces’ do not exist. Ventilation must be maintained during application and continued whilst solvent is released from the paint film during drying.

3.4.14 The ventilation system must prevent the vapour concentration exceeding 10 per cent of the lower explosive limit (or less than this if required by Regulations).

3.4.15 For coatings containing organic solvents, during the drying period an adequate number of air changes must be effected, depending on the type of coating being used. This ventilation should be maintained for at least 48 hours after the application of the system.

3.5.1 Good surface preparation is one of the most important factors governing the performance of a coating. If contaminants such as oil, grease, dirt and chemicals are not removed from the surface they will prevent the adhesion of the coatings. Soluble salts on the surface may lead to osmotic blistering in the coating. Rust left on the surface will loosen, resulting in a loss of adhesion and if mill scale is not completely removed it will cause accelerated corrosion.

3.5.2 Good surface preparation roughens the surface and enables a good mechanical bond to be achieved.

3.5.3 All oil and grease is to be removed from the surface with suitable solvents prior to blast cleaning.

3.5.4 All welded areas and attachments are to be given special attention for the removal of welding flux and weld spatter. Sharp edges should be smoothed and any surface irregularities, including rough weld caps and slag together with rough edges, fins and burrs, should be mechanically treated using power wire brushing, grinding or chipping as appropriate.

3.5.6 The compressed air supply used for blasting is to be free of water and oil and adequate separators and traps are to be provided. Prior to using compressed air, the quality of the air downstream of the separator should be tested by blowing the air on to a clean white blotter or cloth for two minutes to check for any contamination, oil or moisture. This test should be performed at the beginning and end of each shift and at not less than four-hour intervals. If two consecutive tests show no contamination the interval can be extended to once per shift, if subsequent tests show contamination then the four-hour interval is to be reinstated. The test also should be made after any interruption of the air compressor operation. The air should be used only if the test indicates no visible contamination, oil or moisture. If contaminants are evident, the equipment deficiencies should be corrected and the air stream should be retested.

3.5.7 Accumulations of oil and moisture are to be removed by regular purging of the system. Air compressors should not be allowed to work at temperatures in excess of 115°C.

3.5.8 The abrasive used for blasting should be dry and free from dirt, oil or grease and suitable for producing the standard of cleanliness and profile specified. Additionally, any organic or water soluble matter should be a maximum of 0,05 per cent by weight.

3.5.9 Iron or steel abrasives are not normally recommended for in-situ open blasting. If used, careful and thorough cleaning must be carried out to remove all traces of abrasive from the surface.

3.5.10 Although not recommended, recycled grit may be used providing it is correctly graded, dry and free from dirt, oil, grease, organic or water soluble matter. Recirculated grit should be checked for the presence of oil by immersing a sample in water and examining for oil flotation. Tests should be made at the start of blasting, and every four hours until the end of blasting. If compressor operations are interrupted for longer than five minutes, the air supply should be retested prior to use. If oil is evident, the contaminated abrasive should be cleaned or replaced. All surfaces blasted since the last successful test should be completely reblasted.

3.5.11 The amplitude of blast profile from trough to adjacent peak depends upon the type of coating to be applied. The amplitude should be not more than 50 μm for coatings of the zinc silicate type and not more than 75 μm of the high build coatings , unless otherwise specified by the manufacturer. A procedure to measure the surface profile of abrasive blast cleaned steel on site is given in NACE RP 0287.87. The technique utilises a tape that replicates the surface profile and the thickness of the tape is then measured using a micrometer.

3.5.12 Generally, where the final dry film coating is 125 μm or less, it should be in accordance with ISO 8501-1 Sa3 or an equivalent standard, i.e. the surface is to be cleaned to white metal such that a uniformly metallic, slightly roughened surface is produced completely free from foreign matter. Shadowed areas may only be accepted if they are due to differences in the structure of the steel or to a blast cleaning pattern. It should be noted that the possibility of achieving a uniform standard of Sa3 throughout the tanks is remote and a more realistic achievement would be somewhere between Sa2½ and Sa3.

3.5.13 The standard of surface preparation for the majority of the coatings is to be at least in accordance with ISO 8501-1 Sa2½ or an equivalent standard, i.e. the blast cleaned surface is to consist of at least 95 per cent cleaned bare steel and not more than 10 per cent of any single 25 mm square of the surface area is to be discoloured by areas of rust stain or mill scale residues.

3.5.14 In cases where the substrate is corroded or pitted it may be necessary to fresh water wash the areas after abrasive blasting, then reblast, in order to ensure complete removal of soluble corrosion products.

3.5.15 No acid washes or cleaning solutions are to be used on metal surfaces after they have been blasted. This includes inhibitive washes intended to prevent rusting.

3.5.16 Any substandard areas should be identified and must be brought up to the specified standard. Grease free chalk should be used to identify substandard areas and it should be removed after the substandard areas have been rectified.

To confirm that the blasted surfaces are sufficiently dust-free to allow successful coating, they are to be tested in accordance with ISO 8502-3 or an equivalent standard, to an extent and with acceptance criteria defined by the coating manufacturer.

3.5.18 Where surfaces have been coated with a prefabrication primer they are to be similarly cleaned before application of the coatings. If there is extensive breakdown of the primer, the surface affected is to be reblasted.

3.5.19 Since fresh blast cleaned surfaces are subject to immediate corrosion, particularly in areas of high humidity or in a marine atmosphere, it is essential that all cleaned surfaces are coated within four hours of cleaning. In any case, the surfaces are to be coated prior to the end of the working day and before any visible rusting occurs unless humidity can be maintained overnight at a low level.

3.5.20 Checks on the steel surface cleanliness and roughness profile should be carried out at the end of the surface preparation and before the application of the primer and in accordance with the manufacturer’s specifications.

3.5.21 Where abrasive blast cleaning is demonstrated to be impracticable at specific locations, alternative mechanical surface cleaning techniques may be applied. In such circumstances, the surface cleanliness should be in accordance with ISO 8501-1 St3 or an equivalent standard and particular attention must be given to ensuring that the surface profile and soluble salt concentrations are in accordance with the coating manufacturer’s specification.

3.6.1 The composition of any primer used to coat steel after surface preparation and prior to fabrication must be such that it will have no significant deleterious effect on subsequent welding work.

3.6.2 The coatings are to be compatible with any prefabrication primer used and suitable for the intended application.

3.6.3 Materials are to be delivered in original containers with labels intact and the seals unbroken. Containers are to be kept in a safe, clean, well ventilated storage space.

3.6.4 Before use, coatings are to remain unopened in the original containers. Covers are to be kept on opened coating containers when not in use. Coatings are to be used in strict date order and not stored longer than six months unless permitted by the paint manufacturer.

3.6.5 The coating manufacturer’s instructions are to be followed for storage, mixing, thinning and application of coatings along with the recommended time limit between coats and health and safety precautions. Only the thinners recommended by the manufacturer are to be used to thin coatings.

3.6.6 Coatings are to be mixed immediately prior to application. All coating materials are to be thoroughly mixed to give a homogeneous liquid without pigment settling out during application. Mechanical mixers are to be used for all coating mixing operations. The entire contents of the coating container are to be used in mixing to ensure the correct proportion of the base coat and pigment.

3.6.7 Coating material which has livered, discoloured, gelled, or otherwise deteriorated during storage is not to be used. Thixotropic materials which may be stirred to obtain normal consistency may be acceptable.

3.6.8 For coating materials requiring the addition of a catalyst, the pot life under application conditions is to be clearly stated on the label, and this pot life is not to be exceeded. When the pot life limit is reached, the spray pot is to be emptied, material discarded, and new material mixed.

3.6.9 Specification and data sheets on the coating materials are to be available at all times.

3.7.1 The application of a coating should be a well planned activity, integrated in the yard’s construction plans and carried out under controlled conditions to avoid conflicts with other yard operations.

3.7.2 Coatings should be applied in controlled humidity and surface temperature conditions to surfaces which have been blast cleaned to the coating manufacturer’s recommended standard and immediately coated with a compatible prefabrication primer or applied after blast cleaning if this is permitted by the specification.

3.7.3 Areas where the prefabrication primer is damaged in any way may be touched up in accordance with the manufacturer’s specifications.

3.7.4 Each coating layer should have the maximum/minimum thicknesses in accordance with the coating specification. Generally, an 80/20 practice may be adopted which means that 80 per cent of all thickness measurements should be greater than or equal to the nominal dry film thickness (DFT), and none of the remaining 20 per cent is below 80 per cent of the DFT. In the case of tanks (and especially ballast tanks), consideration should be given to adopting a 90/10 practice which means that 90 per cent of all thickness measurements should be greater than or equal to the nominal DFT, and none of the remaining 10 per cent is below 90 per cent of the DFT.

3.7.5 All paints should be applied by airless spray except for stripe coats where brushes or, if recommended by the coating manufacturer as a preferred option, rollers may be used.

3.7.6 Conventional spray may be used for the spraying of zinc silicate tank coatings.

3.7.7 Efficient mechanical stirrers for the correct mixing of paint should be used.

3.7.8 The spray equipment should comply with the paint manufacturer’s recommendations. Adequate moisture traps should be fitted where appropriate so that water or oil can be continuously bled off from the air supply.

3.7.9 Lines and pots are to be thoroughly cleaned before using different materials.

3.7.11 No coating is to be applied if the temperature is below that specified by the coating manufacturer and, in general, the metal surface temperature should be at least 3°C above the dew point before painting is commenced. The temperature, dew point, and relative humidity should be determined with a sling psychrometer. Suitable procedures are given in ASTM E337. Readings are required at the start of work and every four hours.

3.8.2 Where coatings other than the zinc silicate type have been accepted as a single coat application, all welds, edges and any other changes in section may require a stripe coat to be applied.

3.8.3 Successive coats should preferably be of different colours or with a significant shade variation to give contrast and ensure complete coverage of the surface, see also Pt 8, Ch 4, 3.2 General requirements 3.2.3.

3.8.4 All surfaces are to receive the full thickness specified as a minimum. Areas with inadequate coating thickness should receive additional compatible coats until the specified coating thickness is attained. Coatings are to be brushed on to all areas which cannot be properly coated by spray.

3.8.5 Care should be taken to avoid an excessive coating thickness as this could lead to serious consequences, such as solvent and thinner retention, film cracks, gas pockets, etc. Wet coating thickness should be checked during application.

3.8.6 Each coating layer should be adequately cured before application of the next coat, in accordance with coating manufacturer’s recommendations. Intermediate coats must not be contaminated with dirt, grease, dust, salt, over-spray, etc. Job specifications should include the dry-to-re-coat times given by the manufacturer.

3.8.7 Thinners should be limited to those types and quantities recommended by the manufacturer.

3.9.1 Wet film thickness checks should be made as the work progresses using appropriate thickness gauges.

3.9.2 Dry film thickness determinations should be carried out on all significant areas using suitable gauges. (The simple pull-off type gauges are not considered sufficiently accurate for this work.)

3.9.3 The full number of coats specified should be applied and the specified film thickness achieved.

3.9.4 All coatings should be free of pin holes, voids, bubbles and other ‘holidays’. Holiday testing should be carried out using a suitable ‘holiday detector’ set at an appropriate voltage for the coating system.

3.9.5 Any defective areas are to be marked up and appropriate repairs effected. All such repairs are to be rechecked for any uncoated areas.

3.9.7 Damage to coatings is to be repaired by cleaning back to a sound base, recoating the affected areas as required in the specification and feathering to tie with adjoining areas. Prior to the application of any coating, all damage to previous coats is to have been repaired.

3.9.8 The area to be cleaned is to be carried over onto the firm surrounding coating for not less than 25 mm all round the edges. These are to be feathered by a suitable method to ensure continuity of the subsequent repair coating.

3.9.9 Areas with inadequate coating thickness are to be thoroughly cleaned and, if necessary, abraded and, where applicable, additional coats applied until the specification is complied with. These additional coats are to blend in with the final coating at adjoining areas.

3.9.10 Where welding has to take place on coated areas, unless they are approved prefabrication primers the coatings are to be removed locally and the surface after welding is to be prepared and recoated in accordance with the recommended procedures.

3.9.11 When dry film thicknesses are less than those specified, additional coats are to be applied as necessary to achieve specified thickness. For inorganic zinc silicate, areas of low film thickness should not be repaired by additional coats. In this case the coating is to be removed and the area re-coated to the specified thickness or paint manufacturer's recommendation.

3.10.1 It should be noted that paints, coatings and thinners are potentially hazardous from health and safety points of view if not strictly controlled in accordance with good practice. Detailed advice on the safe working practices to be followed should be obtained from the relevant governmental safety agencies.

3.11.1 Maintenance of the corrosion protection system should be included in the overall maintenance schemes.

3.11.2 The most efficient way to preserve the corrosion protection system is to repair any defects found during the in-service inspections (e.g. spot rusting, local breakdown at edges of stiffeners, etc.).

3.11.3 During maintenance hard coatings should be restored using the type originally applied or by a compatible hard coating recognised by LR. The compatibility of coatings should normally be agreed by the paint manufacturer, and the coatings should be applied in accordance with the manufacturer’s requirements.

3.11.4 The restoration of the damaged hard coatings by compatible coatings not recognised by LR will be accepted, provided such coatings are applied and maintained in accordance with the manufacturer’s specification. Details of such coatings are to be reported for information and record purposes.

3.11.5 If the required conditions for the application of the original coating are not achievable, a coating more tolerant of a lower quality of surface treatment, humidity and temperature conditions may be considered, provided that it is applied and maintained in accordance with the manufacturer’s specifications.

3.11.6 Currently there are numerous non-oxidising soft coatings which are being marketed for the purpose of repairing hard coatings. Proposals to use this type of coating, including the manufacturer’s confirmation of their compatibility with the existing coatings, are to be referred for consideration.

3.11.7 It should be noted that soft coatings are, in general, not suitable for use in association with cathodic protection.