Painting over Powder Coated Aluminium
Powder coating became commercially viable in the 1960’s as an alternative to liquid coatings which were considered environmentally damaging. Nowadays, various aluminium items are powder coated in huge numbers and many of these are found in the construction, architectural and interiors industries. Although powder coating is a durable finish, sometimes it fails in that it may chip, bubble up, crack and flake off or the colour may fade over time. In these instances painting over powder coated aluminium is necessary, but in other cases the powder coating may be in perfectly good condition, but in the wrong colour, so a re-spray is still the best option.
What is Powder Coating?
Powder coating is a dry powder that is applied electrostatically to a metal substrate such as aluminium. Once the base metal has been coated in the powder it is heated in an oven which causes the coating to melt. The intense heat transforms the powder elements into a slick and continuous layer that adheres to the aluminium. Often the properties of the powder coating are enhanced through a chemical reaction during the curing process.
Why Paint on to Powder Coated Aluminium?
The beauty of spray-applying a fresh coat rather than re-doing powder coating is that the painting can take place on site which eradicates the inconvenience of having to transport large or immovable objects. Painting over powder coated aluminium saves money too. A re-spray is much cheaper than replacing the items and this new coating protects the aluminium from corrosion as well as improving the appearance.
How to Paint over Powder Coated Aluminium
1. Clean the Substrate
Any dirt or grease on the metal surface will prevent the subsequent coating from clinging, so all contaminates and debris must be removed. The aluminium can be hand washed using a soft brush or jet washed if the aluminium is extremely dirty. Any alkaline degreasing detergent used for cleaning needs to be rinsed off with water and left to dry.
2. Remove Corrosion
The powder coated aluminium may have corroded and the resulting corrosion deposits need to be mechanically stripped off. If powder coating appears chalky this white rust had better be removed and a scouring nylon pad should suffice. A power sander can be used to get rid of any red rust and to reveal a smooth metal surface. Then next stage is to wipe down the substrate with tack cloths to clean off any dust resulting from the removal of corrosion.
3. Abrade the Metal Surface
One of the most difficult obstacles in painting over powder coating is ensuring the new coating adheres to the aluminium. Sanding the substrate removes the gloss from the original powder coating in order to provide a key so that the fresh coating will stick. It is worth noting that anodised aluminium will require more abrading than usual. If parts of the powder coating still remain it is important to feather the edges, so that there aren’t any bumps or ridges on the surface. As with the process of removing corrosion, residues resulting from abrading must then be cleaned off.
4. Mask off Adjacent Surfaces
Mask off the adjacent areas that need to be protected from primer or paint. Plastic sheeting, decorators tape and masking paper can all be used to stop finished surfaces from being ruined through overspray.
5. Prime the Substrate
Once the metalwork is clean and dry it can be primed and then left to cure. This is best done as soon as possible, so that the substrate doesn’t oxidise or get dirty. There are primers appropriate for different alloys, so in this case a primer for aluminium should be used and it is best to consult with professional sprayers. To check whether the primer will adhere to the abraded surface it is a good idea to apply a small amount of it to the aluminium. If it can be wiped off or smears then the aluminium should be abraded further.
6. Spray Apply the Top Coat
There are a range of top coats available on the market to suit the wide variety of objects that can be re-coated. A top coat can be selected in a choice of colours and gloss levels appropriate for painting over powder coated aluminium, but it is crucial to select the right one. The top layer needs to be chosen to adhere to the primer or directly on to the aluminium if a primer hasn’t been used. Again, a professional spray painting company will ensure the most appropriate product is used.