Providing high quality and expert powder coating services across the Midlands and beyond.

In order to get the best out of Powder Coating it is very important to understand that the condition of the surface that Powder Coating is applied to is very important. Like any surface finishing the quality of the finished article is determined by the preparation of the underlying surface.

It is very easy to fall into the trap of applying a coat of powder and ‘covering up surface defects’. This will hide the defects and problems and they may not become apparent for many months or years.

Before addressing Preparation the first and most important issue is the surface or substrate that you are going to Powder Coat. Usually it will be Steel or Aluminium, Castings can be made from Aluminium or Zinc. & Stainless Steel is sometimes coated for cosmetic purposes as well as Brass & Copper parts.

Steel

Steel comes in 3 basic forms;

Hot Rolled Steel;

the steel is rolled to shape and size in its semi molten form. This process leaves a scale on the surface, sometimes referred to as mill scale. It is made up of iron oxides and is about 0.1 mm thick. Although it protects the steel for a while in its raw state it breaks off quite easily along with any coating that is applied to it. Hot Rolled parts should always be pickled or shot blasted before applying a powder coat finish. Some steel mills supply hot rolled material that is Pickled & Oiled after being produced, this removes the scale with an acidic wash and after rinsing it is dipped in light oil. If the hot rolled steel is not pickled and oiled it should always be shot blasted prior to Powder Coating. Otherwise the adhesion of the applied powder coating film to the substrate will only be as good as the adhesion between the iron oxide mill scale and the surface of the steel. Which can result in premature corrosion of the part and costly remedial work or possible failure of the product.

Cold Rolled or Cold Reduced Steel:

In this case Hot Rolled Steel is subjected to further processing that improves the dimensional tolerances and strength of the product. The main issue that relates to Powder Coating & finishing of this type of steel is that the surface of Cold Reduced Steel is smooth and scale free meaning that it can be powder coated without having to remove any scale or surface corrosion.

Zintec:

This is cold reduced sheet steel that has had a very thin coat of zinc applied to the surface. If the product being coated requires long term corrosion resistance then Zintec is the ideal substrate. Although the product costs more than basic cold rolled sheet it can be Powder Coated without any form of pre-treatment thus saving time and money processing the product. Also for products that have unseen areas such as the inside of parts or a reverse side that fits to a wall or to another part then savings can be made because these areas do not need to be coated because the zinc coating will provide basic corrosion resistance, resulting in less powder being used and faster coating times. These savings can in many cases outweigh the extra cost of the Zintec material.

Basic facts; Hot Rolled Steel, usually cheaper but not suitable for powder coating without pickling and/or shot blasting. Cold Rolled steel, dearer than hot rolled but a much better surface for powder coating. Zintec, the best material to powder coat as the thin layer of zinc on the surface provides very good corrosion resistance and it can also provide cost savings due to some areas not requiring coating.

Aluminium

Aluminium is used in a wide variety of industries either in sheet metal, tubular form or other extruded shapes and is usually either anodised or powder coated to protect the surface from corrosion.

Aluminium has a layer of oxide that forms on the surface which can affect the adhesion of the Powder Coated film to the surface this is usually removed by chemical means.

Aluminium Castings

Aluminium is cast in three ways; Sand casting where molten Aluminium is poured into a sand mould to produce a desired shape. Gravity die casting, here again molten Aluminium is poured into a metal mould or die to produce the shape and finally Pressure Die casting where Aluminium is forced under pressure into a metal die. The main issue that can affect Powder Coating is porosity within the metal. When the molten metal is poured (or forced in the case of pressure die-casting) into the die it displaces air that is within the die, this air can get caught up in the general turbulence of the process and become trapped within the resultant die cast part. When the part is powder coated this trapped air can expand and force its way out of the surface of the part causing porosity blisters that can be unsightly and result in parts being rejected. Another common fault is that is caused by less than optimal design of the die. When the molten metal enters the die it is important that it metal flows into the die quickly and spreads into all of the areas of the die before cooling. If cooling of the molten metal happens to early it produces ‘cold spots’ that invariably contain a lot of porosity that cause surface defects when powder coated. There will be further details about this in the Trouble Shooting section later.

Zinc

Is also used to make Die Castings and many of the faults/issues noted above for Aluminium Castings are the same for Zinc Castings.

Powder Coating Stainless Steel

As long as the surface is free from grease, oil or waxes then Stainless Steel can be Powder Coated successfully. It is important to ensure that the parts have the correct curing after coating (i.e. given enough heat/time in the oven). There are many manufactured parts that are normally produced from Stainless Steel and sold in self colour stainless steel, but at some point customers sometimes require the parts coating in a particular colour. An example of this is Flues for boiler or stove chimneys, usually made out of stainless steel but often there is a requirement for them to be coated in Black or other colour. Powder Coatings Ltd have successfully powder coated Stainless Steel Flue Components without any adhesion issues.

Brass and Copper

No problems with powder coating these metals. The main issue again is ensuring the surface is clean and free from Oil, Grease & Waxes. We have successfully coated parts that have a small brass casting attached to a copper disc without any issues for over 20 years. One minor issue can be that pastel shades can show a ‘bleed’ of the copper surface, i.e. the white or pastel shade shows a discolouring when the copper is coated. This does not happen with all powders and can vary with different alloys of copper so sometimes a certain amount of trial & error has to be undertaken to find a powder that does not show the discolouration.