Anodising creates a hard metal oxides coating on aluminium’s surface, protecting aluminium from abrasion or corrosion. Anodised coatings are occasionally damage. While performing anodised repair, first determine whether the base metal is damaged or whether only the metal oxide coating is damaged.
If the base metal coating is damaged, you will need to deanodise the surface, and then repair the base metal by buffing or sanding. In case only the metal oxide coating is damaged, you can use brush anodising, a portable anodising process to repair it.
Wear a lab coat, goggles and chemical resistant gloves as a precaution while working with acids. Strip off the anodised coating in a stripping solution bath, then pour water in a stainless steel bath, heat it by gas firing until the water temperature reaches 140-160 degree Celsius. Now add oxalic and nitric acid until the total volume is around 380 gallons.
You should be able to see through several feet of solution. Add anodised aluminium parts and you’ll see that metal oxides will remove within 15 minutes.
Remove aluminium parts from the bath, rinse with deionised water. Sand or buff to repair gouges and scratches; reanodise, if necessary.
Using a masking tape cover the area around the surface that needs to be repaired. Use sodium hydroxide to remove the damaged metal oxide coating from the surface. Flush away the sodium hydroxide with deionised water.
While applying the cathode rod to the metal surface to be reanodised pass sulphuric acid through a tubular cathode rod. The cathode rod will release positive hydrogen ion charges at the surface, while the surface itself will act as an anode, accumulating metal oxides.
Continue until metal oxides have accumulated to the thickness characteristic of the undamaged surface.