Adding some colour to your galvanised steel structure
Kee Klamp fittings and tube come as standard finish galvanised. When the steel is withdrawn from the galvanizing bath, it has a clean, bright, shiny surface. With time, this changes to a dull grey patina as the surface zinc reacts with oxygen, water and carbon dioxide in the atmosphere to form a complex but tough, stable, protective layer which is tightly adherent to the zinc. The patina takes time to develop and the exact time depends upon the climate around the galvanising. Typically, the time can vary from six months to two years or more.
Whether for aesthetics, additional protection or to provide a ‘not cold to touch finish’ it is often desired to colour coat a Kee Klamp, Kee Access, or KeeGuard project.
- Option 1- Powder-coating
- Option 2- Painting
Powder coating is applied as a free-flowing, dry powder. The main difference between powder coating and a conventional liquid paint is that liquid paint requires a solvent to keep the binder and filler parts in a liquid suspension form.
The powder-coating is applied electrostatically and is then cured under heat to allow it to flow and form a 'skin' to create a hard finish that is tougher than conventional paint.
On colours other than black, we do have a minimum order charge on powder-coating, the minimum order charge covers the cost of the powder and setting up and cleaning.
Alternatively most large towns have businesses that provide powder-coating and as long as you are prepared to wait for a powder-coater to put through another job with the same colour you require they may be able to provide the coating without a minimum charge.
There are four recognised methods of surface pre-treatment that produce a sound substrate for paint coating:
- 1. T-Wash (or its proprietary equivalent)
- 2. Etch primers
- 3. Sweep blasting
- 4. Weathering
Despite the fact that this preparation process has been available for some considerable time, T-Wash is still generally considered to be the best pre-treatment method for painting galvanized steel. T-Wash is a modified zinc phosphate solution which contains a small amount of copper salts. When applied, a dark grey or black discolouration of the zinc surface will result. Any white salt formed by the exposure of the T-Washed surface to moisture must be removed before painting, using a stiff brush. If the T-Washed surface has become contaminated it must be cleaned in accordance with the suppliers’ recommendations.
Constituents of T-Wash
The constituents of T-Wash are phosphoric acid (9.0%), ethyl cellusolve (16.5%), methylated spirit (16.5%), water (57.0%) and copper carbonate (1.0%). Variations to this composition may exist and so it is wise to consult the supplier if a successful result is to be achieved.
Image care of The American Galvanisers Association
Etch primers have also been used successfully. Their major disadvantage is the absence of any visible colour change as is the case with T-Wash. Therefore, there can never be complete confidence that all surfaces have reacted with the primer. Etch primers are most suited to application on older, weathered galvanizing.
A mechanical method of pre-treatment is sweep blasting using fine copper slag, J blast or carborundum powder with a blast pressure of no greater than 40psi (2.7 bar). This will ensure that only the minimum amount of oxide is removed and the zinc surface is left in a slightly roughened condition. Sweep blasting is often used in addition to the chemical preparation stage. Angular iron blasting grit must not be used under any circumstances
image care of The American Galvanisers Association
This process only becomes fully effective after a galvanized surface has been exposed to the atmosphere for a period of at least six months. The surface is prepared using either abrasive pads or a stiff brush to remove all loose adherent materials and making sure that the bright zinc surface is not restored.
This is followed by a hot detergent wash and rinsing with fresh clean water. The surface must be fully dry before any paint is applied. Weathering should not be used as a method of surface preparation in marine environments with high chloride levels.
All paint systems used should be specifically formulated for use on galvanized steel and applied in accordance with the paint manufacturer’s recommendations.
The choices of paint systems will depend upon both the application and service environment. With the decline in the use of chlorinated rubber and alkyd paints, high-build epoxy products and vinyl/vinyl co-polymer systems are increasingly utilised with glass reinforced epoxy being an option for more severe environments. In multi-coat systems the use of micaceous iron oxide (MIO) primer has been shown to give improved adhesion.
Two-pack polyurethane and acrylic urethanes are commonly used as top coats and offer good durability and colour retention. Alternatives include acrylic epoxies and polysiloxanes, the latter offering increased abrasion resistance along with good gloss and colour retention.
At present, high-build epoxies are widely used although water-based products, including polyurethanes, are now being specified. They are less tolerant of poor pre-treatment but their use could continue to grow as the Solvent Emissions Directive becomes more stringent.
We have also found on-line that Hammerite offer a ‘Direct to Galvanised’ paint but we have not had experience of this and would be interested in any feedback in the comments section.