De-oxidising Hard Rubber
Here are three ways that I de-oxidise hard rubber pens...
Technique 1 - Micromesh
This one is all elbow grease and patience! Micromesh is simple and straightforward to use, but it is not suitable for pens that have chasing or imprints because they will be erased as well.
Technique 2 - Chemical Cleaning
Another route is chemically removing the oxidised layer with Mark Hoover's de-oxidising liquid. This works really well with some pens and not so much with others. Be advised - it is messy!
Example 1
Example 2
Example 3
Example 4
Technique 3 - Dye
Yet another option is to use a dye like Pensbury Manor's Pmnr9 dye. This is best for retaining chasing or imprints, but it is tricky to apply in even coatings and it can wear off over time if the pen is not used with care.
Technique 4 - Mix
Finally, some pens might require a mix of techniques...
Example 1
Example 2
