One of my favourite engines in my collection is a little ASP 25 Redhead. As an old engine it’s in excellent condition but has suffered some minor hanger rash in the form of scratching to the back of that distinctive red cylinder head. Anodising applies a tough natural oxide surface layer through electrolytic passivation in an acid bath. Not a process typically performed at home and not a process that can be ‘repaired’ easily without treating the affected part to the entire process again.
The good news is there is a simple quick-fix that is successful enough at hiding scratches to such an extent that at first glance the observer may be none the wiser. It’s also very cheap; all that is required is a permanent marker pen! Surface scratches almost disappear but deeper marks are a little more visible: more due to shadows cast by raised edges than anything else. Anodising in this hobby is typically coloured and these pens come in just about any colour you want. The pen only needs to be a close enough match for the repair to work well as shown with my engine below: