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Hot dip galvanising alone is a cost efficient, long-lasting corrosion protection solution. But, it is also possible to paint over galvanised steel. People do this for a multitude of reasons. Examples include providing more protection in aggressive settings. Another would be increasing a structure’s economic life. Adding colour for safety, camouflage, or aesthetic reasons are common too.
In this post, we want to talk more in detail about painting the steel.
Taking the right steps
Painting galvanised steel is an easy affair as long as you take the correct steps. The first thing you must do is prepare the metal. Some reliable methods of surface pre-treatment exist. They generate a decent surface for paint to adhere to. They include weathering, sweep blasting, etch primers, and t wash. You can use them on ERW tubes and other steel products.
Afterwards, you must select the correct paint for your galvanised metal. Every paint system needs to be formulated specifically for this use. You have to apply it in accordance with the manufacturer’s recommendations too. This includes details like curing rates, dry film thickness, and time of application.
Guidelines for pre-treatment
There are some great instructions for painting galvanised steel. They are the result of a study by an independent research centre and one prime UK paint manufacturer. It looked into the performance of commercially available paints and pre-treatment systems. Also, it took a look at the parameters influencing their performance on items that had hot dip galvanisation.
It is true that the pre-treatment should be performed straight after galvanising. This will be before the exterior has time to suffer contamination in any way. But, this isn’t always practical. You are able to perform pre-treatment later, however you must clean the surface to dispose of all contaminants. Examples include dirt, grease, and oil. The cleaning work also cannot leave any residues on the exterior.
Some recognised methods exist for surface pre-treatments. We want to look at a few of the main ones now so you know options for ERW tubes and more.
T-Wash
This process has been around for quite some time. Despite that, it is still generally thought of as the leading pre-treatment method for painting galvanised steel. T-Wash is an altered zinc phosphate solution. It holds a small degree of copper salts. When you apply it, a black or dark grey discolouration of the zinc exterior will happen.
One thing to note here is you can’t allow T-Wash to pool on horizontal exteriors. Otherwise, this will stop maximum paint adhesion. If there is excess, it is best to remove it with water.
T-Wash is most appropriate for application to new galvanisation. You should not use it on weathered materials. It is also necessary to allow for enough time for the T-Wash to react and dry. Only after can you apply the first coat of paint.
Etch primers
You can use these successfully to prime the steel. Their only major con is the lack of any visible colour change like with T-Wash. Thus, users can’t have full confidence every surface has reacted to the primer. Etch primers are most appropriate for applications on weathered, older materials.
Sweep blasting
Here we have a mechanical method of pre-treatment. It involves the use of a fine copper slag. This will guarantee the removal of only the minimum level of oxide. The zinc exterior will remain in a slightly rough state.
You should take care when sweep blasting extremely thick coatings to stop coating damage. The optimum nozzle-to-work piece angle of blasting and distance should be chosen. This is for every exterior if you are aiming for the finest results.
A lot of people combine this method with a chemical preparation phase. It can improve the quality.
We give you unbeatable choice for ERW tubes
At Union Steel Tubes, we always want our clients to have optimum choice with our tube. Besides normal options like round, square, rectangular, and oval, we have other variants. These are ones like seamless and pre-galvanised. The latter are ones you can then choose to paint if you need to.
We make sure that clients only receive the best products, whether it is ERW tubes or seamless. So, if you’d like to shop with us, please get in touch.
