" Larry's Tech Corner " Post #2
Paint Adhesion on Anodized Aluminum
The anodic coating itself is an excellent base for applying organic coatings. “Organic coatings” include products like epoxies, polyurethanes, alkyd enamels, primers, etc. The ideal paint base is an unsealed anodized coating and it’s best to apply the primer and/or the top coat within 24 hours of anodizing, if possible.
When anodized parts are sealed and then painted, is when the problem of adhesion of the organic coating is prevalent. This is because most Type II and Type III anodic coatings are sealed in what is referred to a “proprietary, mid-temp, nickel acetate seal”. These products contain surfactants, often called “wetting agents”. Surfactants are slippery and “paint” doesn’t adhere very well to slippery surfaces.
There are three common methods for treating anodized surfaces that will subsequently have some type of organic coating applied:
Leave the anodized parts unsealed. This includes not dipping them in any of the tanks after a proprietary, mid-temp, nickel seal. There are carry-over surfactants in those tanks, too.
Seal the parts in high quality deionized (DI) water ( < 5 µmS conductance) with no additives except a pH buffer such as sodium acetate. The bath should be near boiling for a good seal (> 205oF).
Use a duplex seal. The first step is straight nickel acetate at 5 g/l in DI water, 195oF to 205oF, 5 minutes. This is followed by sealing in DI water as per above.
Handle any parts to be painted using clean, white cotton gloves. Protect the parts from dust accumulation. Paint as soon as possible.