Preparing for coating /polishing

To prepare hot-dip galvanised material for the application of a powder coating, the material has to be polished or prepared for a coating after galvanising. Impurities and imperfections are removed by sanding and polishing, while keeping the zinc layer intact. Not all irregularities, such as weld spatter and large bulges, can be removed due to the risk of damage to the zinc layer.

Rotocoat distinguishes between three levels of quality of preparation for coating. They differ in terms of the final quality with respect to appearance, corrosion protection and cost:

• low          : industrial coating preparation;
• average  : decorative coating preparation;
• high         : architectural coating preparation (highest aesthetic standard).

Choice of quality level for coating preparation/polishing

When selecting the quality of the coating preparation, it is important to determine what the product will be used for and the requirements that the product will have to meet in terms of protection from corrosion and of appearance.

When appearance is less important, industrial or decorative coating preparation will be adequate. When the appearance of the product is essential, architectural coating preparation is an option.

The choice of quality level for preparation affects not only the appearance, but also the level of protection from corrosion. As more polishing is required, there is an increased risk of damage to the zinc layer and a reduction in the level of protection. There are also cost considerations. More polishing means that Rotocoat will charge more.

In architectural coating preparation, the aesthetic requirements predominate and the level of protection from corrosion is of secondary importance. In consultation with, and after approval from, the client, additional polishing can take place in which the zinc layer can, and may, be damaged to such an extent that it no longer fulfils the requirements for hot-dip galvanising under NEN-EN-ISO 1461 and Rotocoat will no longer be able to guarantee the product. Rotocoat engages in architectural coating preparation only on the basis of agreements made in advance with the client and a limit sample to be defined accurately with the client. 

1. Industrial coating preparation

The lowest quality of coating preparation is the industrial approach, which addresses only the features resulting directly from hot-dip galvanising such as protrusions and zinc ash.

This approach is used widely on solid objects made of structural steel such as bars, beam, strips and sheets, and Rotocoat implements it to the requirements of NEN-EN-ISO 1461. 

In short, this means that

• All protrusions that may cause injury are removed or rounded off.
• Zinc ash is removed.
• Any non-galvanised areas in the zinc layer are not repaired with zinc compound because zinc compound does not always form a homogeneous layer with the powder coating to be applied later.
• There is no grinding of the surface and it remains in its original condition.
• Only larger hard zinc spots are removed.
• Roller traces and splits are not removed.
• In order to fully maintain the protective action of the zinc against corrosion, weld seams which generally rise up during hot-dip galvanising as a result of additional zinc build-up will not be sanded smooth.

Requirements for industrial coating preparation

To ensure that the powder can bond well to the object and that the coating is thick enough everywhere, objects due for industrial coating preparation have to comply with the following requirements:

• The sharp edges are removed by deburring or edge breaking, and laser-cut edges are rounded off.
• No irregularities (roughness, the presence of seams, amount of rust, dullness) will be visible on the steel surface.
• The objects will preferably be made from a steel with a low silicon content (<0.04%), or a silicon content between 0.14% and 0.25% (see also: http://rotocoat.nl/verzinken/technische-informatie/invloed-staalsamenstelling-op-zinklaag) because it is only then that relatively smooth and thin layers can be produced during hot-dip galvanising.

Objects powder coated after industrial coating preparation meet the requirements for corrosion protection but they do not comply with stringent optical requirements. Spots, pores, zinc drops and irregularities in the surface will be visible. 

2. Decorative coating preparation

The most frequently selected quality level for coating preparation is decorative coating preparation, which complies with the requirements for both corrosion protection and decoration.

Industrial coating preparation removes or rounds off only the features caused directly by hot-dip galvanising. Decorative coating preparation results in a smoother surface and irregularities such as hard zinc spots and zinc drops are removed. The surface can be made smoother only in places that can be machined easily. 

For example, the outside of square pipes and tubes can only be smoothed by grinding to a limited extent. Because they have no straight surfaces and the edges are round, sanding can wear away some places faster and damage the zinc layer. The same is true for rolled profiles which, in similar forms, also have no straight surfaces. In addition, non-galvanised steel has its own structure which, even after hot-dip galvanising, remains visible and cannot be 'polished away' without affecting the protective zinc layer.

The work done for decorative coating preparation must comply with the applicable standard, NEN-EN-ISO 1461. Because decorative coating preparation is a step further in terms of the finishing of the surface as a whole, there is an increased risk that corrosion protection will be reduced by comparison with objects that have undergone industrial coating preparation. The client will have to accept that the thickness of the zinc layer may be thinner than the standard locally, as long as the thickness of the layer is at least 50% of the thickness required by the standard.

When the layer is thicker due to the use of reactive steel during galvanising (see also http://rotocoat.nl/verzinken/technische-informatie/invloed-staalsamenstelling-op-zinklaag), it can be sanded to only a limited extent in order to obtain a 'smooth' surface. These layers are not covered by the NEN-EN-ISO 1461 standard. In these cases, Rotocoat will make separate agreements with the client and the coating preparation will be completed only against payment of the agreed amount of work.

To ensure a uniform appearance for a project, it is important to order steel from one and the same steel trader and to require the ordered steel components to come from the same production batch in order to increase the probability of a uniform galvanising result and therefore a uniform duplex result.

Requirements for decorative coating preparation

Objects destined for decorative coating preparation must comply with the following conditions:

• The sharp edges are removed by deburring or edge breaking and laser-cut edges are rounded off.
• The objects may not contain any reactive steel (see also Effect of steel composition on zinc coating).
• The object should be designed so that it has enough suspension holes in places that can be machined properly and are preferably not in sight.
• The object must be free of any rolling and other surface flaws. When these flaws occur more frequently than normal (more than one flaw per dm2), specific agreements must be made with clients.

Work on decorative coating preparation includes:

• complete grinding/sanding of the surface and removing all zinc spots and zinc drops, in so far as the structure permits and if this can be done by machine;
• smoothing non-galvanised areas;
• sanding of zinc and suspension points.

Raised welds and material defects will not be smoothed.

3. Architectural coating preparation

The highest quality of coating preparation is architectural coating preparation. This is the most intensive approach to surface treatment before coating, in which corrosion protection is subordinate to visual requirements. This means that these objects no longer comply with the applicable standard, NEN-EN-ISO 1461. In architectural coating preparation, the object is subjected to full grinding and the surface is made smooth. Any welds are also smoothed away and slight irregularities are, if necessary, ground away down to top of the substrate. Rotocoat engages in architectural coating preparation on the basis of agreements made in advance with the client and on the basis of prior agreements with the client and a limit sample to be defined accurately with the client.

Objects destined for architectural coating preparation must comply with the following conditions:

• The sharp edges are removed by deburring or edge breaking and laser-cut edges are rounded off.
• The objects may not contain any reactive steel (see also Effect of steel composition on zinc coating).
• The object should be designed so that it has enough suspension holes in places that can be machined properly and are preferably not in sight.
• The object must be free of any rolling and other surface flaws. When these flaws occur more frequently than normal (more than one flaw per dm2), specific agreements must be made with clients.