how do i wash micarta handles?

Well it's the fact that those scales are "raw", meaning they aren't waxed or sealed. They are more grippy in the raw state, but they collect grime. Washing with soap takes off lots of it, but the "pores" get clogged.

Sanding the stains works, but smooths out the top layer of fabric, and that's where the dirt is. Another option is to oil or wax them, buff them, or dye them black.

I've not done it, but a light sand blasting might leave them clean and grippy, but how often can you do that before the shape is changed till they just don't fit right.

I'd just get them dirty and leave it be. I've oiled mine, just to make them dark to start with, it still washes off.
 
Scrub them with a thick paste of water, dish detergent, and baking soda.
When clean and dry, wax with paste wax.
 
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