You can directly clean the Ultem & PEEK parts with acetone.
If you feel like the acetone is too strong, you can dilute it with water.
Diluting acetone is also more economical and creates less odor.
The Ultem & PEEK parts have superb chemical resistance.
Unlike the typical plastics used for most of the fountain pens, acetone will not react chemically with Ultem & PEEK, even if it is exposed for a long period of time.
Also, Ultem is impervious to ink stain unlike general transparent plastics sych as: acrylic (also called as PMMA / plexiglass / resin), polycarbonate, etc.
This is the reason why typical transparent plastics are not used for Eureka pen parts that require transparency.
