I’ve modified several grips. I’ve used a knife, small belt sander (knife sharpener), the sanding drum on a Dremel tool, a course block sander and a combination thereof.
Careful with the knife – easy to make deep cuts that won’t sand out without getting into white plastic shell. The belt sander and Dremel make a real mess, throwing black debris particles everywhere. The block sander is slow, but a bit more precise and less messy. I’m not particularly handy and tend to be impatient and heavy handed so I go to fast and end up with not so pretty outcomes. At the bottom of this post are some closer looks at the grip mods and consequeces of impatience.
The grips are comprised of a hard molded plastic shell (white in color) to which the soft rubber (black) is bonded. You can remove as much material as you want but will eventually run into the white plastic. Doesn’t hurt anything except aesthetics. There is a fair amount of rubber material in the finger groves and on the bottom of the butt.
If aesthetics are important to you, JUST GO SLOW and use the belt sander/sanding block course to fine.
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Paul