Advantages Of Blast Cleaning
It has been indicated by The British Iron & Steel Research Association that surface coating applied to blast cleaned substrates may be expected to last five times as long as the same surface that had been weathered, then manually wire brushed.
The preparation of steelwork obviously has a bearing on the success or failure of any surface coating, and therefore the following information is relevant to steelwork prepared to Swedish Standard SIS 05 59 00 – Sa 2.5.
British Standard BS 7079: Part A1: 1989 (ISO 8501 – 1:1988) – Surface Finish of Blast Cleaned Steel for Painting and Steel Structures Painting Council of America also set out standards for cleanliness.
Amplitude – Surface Profile.
Amplitude is a measurement (given in microns), that shows the difference between the peaks and troughs in a metal surface produced by blast cleaning. There must be adequate amplitude on the surface of the substrate to ensure good adhesion. However, if the amplitude on the surface is too rough, then there is a risk that the peaks of the blast cleaning process will protrude through the paint film, leading to significantly higher paint consumption or ‘peak rashing’ or spot rusting.
On average, a surface that has been grit blasted should have a profile that lies between 50 to 70 microns, and shot blasted steel under blast primers should be 30 to 50 microns. Profiles in excess of 100 microns should be avoided.
Any surface that is to be blast cleaned and subsequently painted must be thoroughly degreased and cleaned beforehand. This should be done using products in the HMG Prep Clean Range.
Degreasers must be used in conjunction with an absorbent cloth which must be changed frequently to avoid re-depositing oil / grease onto the substrate.
When degreasing, it is necessary to wear adequate personal protection equipment. Refer to Material Safety Datasheets for further information.
If a surface coating is damaged during the manufacturing process, in transit or when being handled, it will be necessary to repair the damaged area. If it is at all possible, the original surface treatments should be used in the repair process, to the original specification.
Surface coatings that are damaged which are being rectified in shop must be blast cleaned to original standard. Particular attention should be paid to weld areas where spatter, slag, etc and heat damaged coatings should be removed. All areas to be treated should be cleaned of all contamination including oil, grease dirt and other foreign matter. Subsequent blast cleaning should overlap existing sound coatings.
Prior to the use of any site applied products, all shop applied coatings must be examined for cleanliness and damage. Damage must be repaired to in-shop specifications and all areas must be thoroughly cleaned before further coatings are applied. Particular attention must be made to ensure that any soluble salt contamination is completely removed.
All surface contamination including oil, grease and lubricant must be thoroughly removed from the surface before the substrate is abraded and subsequently painted. Removal of foreign matter can be done using a high pressure hot water detergent wash and rinse, but removal of oil, grease and lubricants must be done with a cleaning solvent from HMG Prep Clean range.
Abrasion of the aluminium surface is also required to assist the adhesion of subsequent coatings.
The application of the finished item will determine the grade of aluminium and therefore will determine the abrasion required on the metal surface. For light grades of aluminium, Scotch pads are suitable, (varying grades of pad are available). However if the grade of aluminium is heavier or if box section is to be coated then 80 – 120 grit aluminium oxide pads, used in conjunction with an orbital sander. Any residual aluminium powder created by abrading must be removed prior to the application of surface coatings.
Structural aluminium for use in such application as bridges, shipping and flooring may require specialist applications.
Enquiries should be made to HMG Laboratory before application of surface coatings.
All surfaces must be thoroughly cleaned, removing all dirt, grease and any other deposits. To ensure that no moisture is present in the wood, it should be well seasoned and it is advisable to apply of a suitable wood primer before the surface is exposed to external elements. As a guideline, there should be no more than 18% moisture content within the wood before any coatings are applied.
If a wood preservative treatment has previously been applied to the surface, then details of that product should be supplied to the HMG Laboratory in order to obtain a system of coating that would be most suited.
If there are knots in the wood then they should be treated with HMG Heavy Duty Shellac Sealer, to ensure that there is no resinous bleed from the wood after coating.
The woodwork should be abraded with a suitable grade of wet or dry abrasive paper to ensure a smooth surface for painting. Each coating should be abraded prior to subsequent coatings are applied.
After a primer has been applied, if there are any nail holes cracks or other surface defects, a suitable HMG wood filler should be applied.
Note – Cellulose fillers should not be used on exterior woodwork or over synthetic primers.
The woodwork surface should always be fully moisture free and clean prior to painting and between coats to ensure full adhesion.
Any wooden surfaces that will be in constant contact with brickwork stone or plaster, such as doorframes or walls, should have at least two coats of suitable primer, paying particular attention to end grains, to give added protection against moisture penetration from surrounding surfaces. Application of the primer should be carried out using a brush and worked well in to ensure full coverage to exposed end grain.
Masonry, Concrete And Porous Surfaces
Any surface contamination, such as dirt, grease and loose flaking or defective material should be removed. It is very important to ensure that these surfaces are fully dried our prior to application of any surface coatings and are free of surface damp, as failure to ensure this will seriously impair adhesion of surface coatings.
Concrete must have been applied for a minimum of 8 weeks old with maximum moisture content of 4%. Each individual concrete surface should be assessed before treatment to evaluate the most suitable method of surface preparation and the most suitable coatings.
It is possible that there may be some residual salts left behind after the concrete has dried, this is otherwise known as Laitance or Efflorescence. This must be removed using a good quality concrete cleaner. Additionally, controlled blast cleaning may be required, but this must be done professionally, to ensure that concrete aggregate is not exposed.
Any other cracks or imperfections should be made good by cutting out and filling with a suitable hard plaster, sand/cement mix or approved filler as appropriate. Cellulose fillers are not suitable for use in exterior situations.
Large cracks and other defects should be cut out and filled using an approved cement filler. All nibs must be removed to ensure that there are no breakages in subsequent paint films.
Previously Painted Surfaces
All previously painted surfaces must be thoroughly prepared removing all rust, dirt, oil, grease, loose flaking or damaged existing paint.
Any existing coating that is to remain must be thoroughly cleaned and abraded to allow subsequent coatings a key to adhere to. All existing coatings must be sound, hard and well adhered to the substrate.
Where bare substrate is visible, the area should be spot primed. This primer should overlap the existing paintwork by at least 1 inch and edges of the existing paint should be feathered to ensure an even application of new coating.
If the existing paintwork type or manufacturer is not known then the HMG Laboratory should be contacted for further advise to ensure that problems of incompatibility are not encountered. It is also prudent to effect a test for compatibility before overcoating the entire surface.
As a rule of thumb, the protection and durability of a surface coating system normally increases in relation to the total film thickness. When specifying a paint system, the nominal dry film thickness (DFT) for each coating and for the full recommended system.
If less paint is applied then the total protection and durability will be adversely affected.
Variations in film thickness during application are to be expected, but can monitored in the following ways:
- Control of wet film thickness (WFT) during application.
- Measurement of dry film thickness (DFT) on completion.
Dry film thickness should be established as follows:
- On 1m² an average film thickness should be taken and shall be equal or above the required nominal dry film thickness (DFT). The total DFT must never be less than 75% of the total nominal coating thickness.
- No less than 5 readings will be taken to give an average DFT on an area coated, and no less than 5 readings on an area greater than 1m².