Diagnosis
The woven fabric cover acts as a dust magnet. Using liquids, compressed air, or rough brushes will push dirt deeper or stain the material.
How to Fix It
Use a soft, dry microfiber cloth—the kind for eyeglasses or screens. Gently wipe in one direction, following the weave of the fabric. For stubborn spots, slightly dampen a corner of the cloth with distilled water, wring it out completely, and dab (don't rub) the spot.
If That Didn't Work
For ingrained dust, use a soft-bristled brush like a clean makeup brush or a new, soft paintbrush. Gently brush the fabric in one direction to lift the dust, then use the dry microfiber to pick it up.
Heads Up
Never use vacuum cleaner attachments, household cleaners, isopropyl alcohol, or steam. The moisture can seep through to the internal electronics and the chemicals will discolor the fabric permanently.