In my experience,dry sanding plastic results in a fuzzier surface .
Whether to sand is your choice,
If you are going to sand,I suggest a wet or dry paper,wet.A 400 or 500 grit will do.
To clean up after that,any one of the following might be worth trying:
Very fine steel wool(or a tooth brush) charged with Bon Ami,wet.
A scotchbrite pad,maroon,might work well.
One of the Novus plastic polishing compounds.Automotive rubbing compound would be an alternative.
Even a very light buffing on a soft cloth wheel charged with a fine white compound.
I have never worked on a Glock frame or done what you are trying to do.
I have worked as a modelmaker .
Try any of it,gently,and observe the results.
I think everything I mentioned would do what you are trying to do.