What type of plastic coating is it? polyesther etc
As the term (powder coat) imlies the plastic goes on as a powder then is heated to cure the plastic coating.
http://www.powdercoatingonline.com/html/powdercharacteristics.html
Anodizing only really works well on new (scratch free surfaces) parts.
http://www.hard-anodising.co.uk/the-anodising-process.asp
"As all anodised coatings are derived from oxidation of the metal surface, the alloy composition and heat treatment condition have a large effect upon the resultant coating. The anodic film has excellent adhesion to the substrate, as it is an integral part of the structure, in contrast to, say, a painted or electroplated component. Anodised coatings provide an enhancement in properties over the base material particularly in respect to wear, corrosion, temperature resistance and electrical insulation. Generally speaking, the surface finish of the original work piece is reproduced, with slight roughening; there is no effect. Any defects such as corrosion or polishing 'burns' will be emphasised rather than hidden."